diff --git a/.github/workflows/install-scripts.yml b/.github/workflows/install-scripts.yml
index 889d13c..3dbdc80 100644
--- a/.github/workflows/install-scripts.yml
+++ b/.github/workflows/install-scripts.yml
@@ -8,12 +8,14 @@ on:
       - install.sh
       - install.ps1
       - tests/install_sh_sudo_test.sh
+      - tests/install_ps1_test.ps1
       - .github/workflows/install-scripts.yml
   pull_request:
     paths:
       - install.sh
       - install.ps1
       - tests/install_sh_sudo_test.sh
+      - tests/install_ps1_test.ps1
       - .github/workflows/install-scripts.yml
 
 permissions:
@@ -34,6 +36,23 @@ jobs:
       - name: installer behavior tests
         run: sh tests/install_sh_sudo_test.sh
 
+  windows:
+    name: install.ps1 behavior (windows)
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v7
+      - uses: actions/setup-go@v7
+        with:
+          go-version-file: "go.mod"
+          cache: false
+      - name: installer behavior tests (Windows PowerShell 5.1)
+        shell: powershell
+        run: ./tests/install_ps1_test.ps1
+      - name: installer behavior tests (PowerShell 7)
+        if: ${{ !cancelled() }}
+        shell: pwsh
+        run: ./tests/install_ps1_test.ps1
+
   mirror:
     name: mirror install scripts
     runs-on: ubuntu-latest
diff --git a/install.ps1 b/install.ps1
index 96a1827..fde7207 100644
--- a/install.ps1
+++ b/install.ps1
@@ -156,11 +156,13 @@ try {
 
     $DestPath = Join-Path $InstallDir $InstalledName
     if (Test-Path $DestPath) {
-        # A running .exe can't be overwritten but can be renamed: move it aside
-        # so `update` can replace the binary it is running from.
-        $OldPath = "$DestPath.old"
-        Remove-Item -Path $OldPath -Force -ErrorAction SilentlyContinue
-        Move-Item -Path $DestPath -Destination $OldPath -Force
+        # A running .exe can't be overwritten or deleted but can be renamed:
+        # move it aside under a unique name so `update` can replace the binary
+        # it is running from. Copies moved aside earlier are deleted; one that
+        # is still running stays locked and is left for a later install.
+        Get-ChildItem -Path $InstallDir -Filter "$InstalledName.*.old" -File |
+            Remove-Item -Force -ErrorAction SilentlyContinue
+        Move-Item -Path $DestPath -Destination "$DestPath.$([System.Guid]::NewGuid().ToString('N')).old"
     }
     Move-Item -Path $BinaryPath -Destination $DestPath -Force
 
diff --git a/tests/install_ps1_test.ps1 b/tests/install_ps1_test.ps1
new file mode 100644
index 0000000..353d532
--- /dev/null
+++ b/tests/install_ps1_test.ps1
@@ -0,0 +1,171 @@
+# Behavior tests for install.ps1. Hermetic: release archives are built locally
+# from a stub program and served by an Invoke-WebRequest stand-in, so no
+# network is touched. Needs Windows and Go on PATH.
+$ErrorActionPreference = "Stop"
+
+$Root = Split-Path -Parent $PSScriptRoot
+$InstallScript = Join-Path $Root "install.ps1"
+$TmpDir = Join-Path ([System.IO.Path]::GetTempPath()) "install-ps1-test-$([System.Guid]::NewGuid().ToString('N'))"
+$Fixtures = Join-Path $TmpDir "fixtures"
+New-Item -ItemType Directory -Path $Fixtures -Force | Out-Null
+
+$Arch = switch ($env:PROCESSOR_ARCHITECTURE) {
+    "AMD64" { "x86_64" }
+    "ARM64" { "arm64" }
+    default { throw "unsupported architecture: $env:PROCESSOR_ARCHITECTURE" }
+}
+$Archive = "flashduty-cli_Windows_${Arch}.zip"
+
+# Stub CLI: prints its build version; `sleep` keeps it running so the
+# installed .exe is locked while a reinstall happens.
+$StubSource = Join-Path $TmpDir "stub.go"
+Set-Content -Path $StubSource -Encoding ascii -Value @'
+package main
+
+import (
+	"fmt"
+	"os"
+	"time"
+)
+
+var version = "dev"
+
+func main() {
+	if len(os.Args) > 1 && os.Args[1] == "sleep" {
+		time.Sleep(10 * time.Minute)
+	}
+	fmt.Println("stub " + version)
+}
+'@
+
+# Each release is <fixtures>/<version>/{archive, checksums.txt}, the layout
+# install.ps1 downloads from <MIRROR_URL>/releases/download/<version>/.
+function New-Release($Version) {
+    $dir = Join-Path $Fixtures $Version
+    $build = Join-Path $dir "build"
+    New-Item -ItemType Directory -Path $build -Force | Out-Null
+    & go build -ldflags "-X main.version=$Version" -o (Join-Path $build "flashduty-cli.exe") $StubSource
+    if ($LASTEXITCODE -ne 0) { throw "go build failed for $Version" }
+    $zip = Join-Path $dir $Archive
+    Compress-Archive -Path (Join-Path $build "flashduty-cli.exe") -DestinationPath $zip -Force
+    $sum = (Get-FileHash -Path $zip -Algorithm SHA256).Hash.ToLower()
+    Set-Content -Path (Join-Path $dir "checksums.txt") -Encoding ascii -Value "$sum  $Archive"
+}
+
+# Stand-in for the cmdlet: functions take precedence over cmdlets, so the
+# installer run below in this session downloads from the local fixtures.
+function Invoke-WebRequest {
+    param([string]$Uri, [string]$OutFile, [switch]$UseBasicParsing)
+    if ($Uri -notmatch '/releases/download/([^/]+)/([^/]+)$') { throw "unexpected URL: $Uri" }
+    $src = Join-Path (Join-Path $Fixtures $Matches[1]) $Matches[2]
+    if (-not (Test-Path $src)) { throw "no fixture for URL: $Uri" }
+    Copy-Item -Path $src -Destination $OutFile -Force
+}
+
+function Invoke-Installer($InstallDir, $Version, $InstalledName) {
+    $env:MIRROR_URL = "https://mirror.example/flashduty-cli"
+    $env:FLASHDUTY_VERSION = $Version
+    $env:FLASHDUTY_INSTALL_DIR = $InstallDir
+    $env:INSTALLED_NAME = $InstalledName
+    & $InstallScript | Out-Null
+}
+
+function Assert-Runs($Exe, $Version) {
+    $out = (& $Exe | Out-String).Trim()
+    if ($out -ne "stub $Version") { throw "$Exe printed '$out', want 'stub $Version'" }
+}
+
+function Assert-ExactName($Dir, $Name) {
+    $names = @(Get-ChildItem -Path $Dir -File | Where-Object { $_.Name -notlike "*.old" } | ForEach-Object { $_.Name })
+    if ($names.Count -ne 1 -or $names[0] -cne $Name) {
+        throw "files in ${Dir}: [$($names -join ', ')], want exactly [$Name]"
+    }
+}
+
+function Get-OldCopies($Dir, $Name) {
+    @(Get-ChildItem -Path $Dir -File -Filter "$Name*.old")
+}
+
+function Test-Case($Name, [scriptblock]$Body) {
+    try {
+        & $Body
+        Write-Host "PASS: $Name"
+    } catch {
+        Write-Host "FAIL: $Name -- $_"
+        $script:Failed++
+    }
+}
+
+$Failed = 0
+$Sleepers = @()
+$SavedUserPath = [Environment]::GetEnvironmentVariable("Path", "User")
+try {
+    New-Release "v1.0.0"
+    New-Release "v2.0.0"
+    New-Release "v3.0.0"
+
+    Test-Case "default name installs flashduty.exe" {
+        $dir = Join-Path $TmpDir "default"
+        Invoke-Installer $dir "v1.0.0" ""
+        Assert-ExactName $dir "flashduty.exe"
+        Assert-Runs (Join-Path $dir "flashduty.exe") "v1.0.0"
+    }
+
+    Test-Case "INSTALLED_NAME=fduty installs fduty.exe" {
+        $dir = Join-Path $TmpDir "fduty"
+        Invoke-Installer $dir "v1.0.0" "fduty"
+        Assert-ExactName $dir "fduty.exe"
+        Assert-Runs (Join-Path $dir "fduty.exe") "v1.0.0"
+    }
+
+    Test-Case "INSTALLED_NAME=FDUTY.EXE installs FDUTY.exe" {
+        $dir = Join-Path $TmpDir "fduty-upper"
+        Invoke-Installer $dir "v1.0.0" "FDUTY.EXE"
+        Assert-ExactName $dir "FDUTY.exe"
+        Assert-Runs (Join-Path $dir "FDUTY.exe") "v1.0.0"
+    }
+
+    $runDir = Join-Path $TmpDir "running"
+    $runExe = Join-Path $runDir "flashduty.exe"
+
+    Test-Case "reinstall replaces the installed exe while it is running" {
+        Invoke-Installer $runDir "v1.0.0" ""
+        $script:Sleepers += Start-Process -FilePath $runExe -ArgumentList "sleep" -PassThru -WindowStyle Hidden
+        Start-Sleep -Seconds 1
+        Invoke-Installer $runDir "v2.0.0" ""
+        Assert-Runs $runExe "v2.0.0"
+        if ($script:Sleepers[-1].HasExited) { throw "running v1.0.0 process exited during reinstall" }
+        if ((Get-OldCopies $runDir "flashduty.exe").Count -ne 1) { throw "want one moved-aside copy of the running exe" }
+    }
+
+    Test-Case "reinstall while the moved-aside copy is still running" {
+        # The v1.0.0 process from the previous case still runs from the
+        # moved-aside file, which therefore cannot be deleted or replaced.
+        $script:Sleepers += Start-Process -FilePath $runExe -ArgumentList "sleep" -PassThru -WindowStyle Hidden
+        Start-Sleep -Seconds 1
+        Invoke-Installer $runDir "v3.0.0" ""
+        Assert-Runs $runExe "v3.0.0"
+        Assert-ExactName $runDir "flashduty.exe"
+    }
+
+    Test-Case "a stale moved-aside copy from an earlier install is cleaned up" {
+        $dir = Join-Path $TmpDir "stale"
+        Invoke-Installer $dir "v1.0.0" ""
+        Invoke-Installer $dir "v2.0.0" ""
+        Invoke-Installer $dir "v3.0.0" ""
+        Assert-Runs (Join-Path $dir "flashduty.exe") "v3.0.0"
+        $old = Get-OldCopies $dir "flashduty.exe"
+        if ($old.Count -ne 1) { throw "want one moved-aside copy after three installs, got $($old.Count): $($old.Name -join ', ')" }
+    }
+} finally {
+    $Sleepers | Where-Object { -not $_.HasExited } | Stop-Process -Force -ErrorAction SilentlyContinue
+    [Environment]::SetEnvironmentVariable("Path", $SavedUserPath, "User")
+    Start-Sleep -Seconds 1
+    Remove-Item -Path $TmpDir -Recurse -Force -ErrorAction SilentlyContinue
+}
+
+if ($Failed -gt 0) {
+    Write-Host "$Failed case(s) failed"
+    exit 1
+}
+Write-Host "all install.ps1 cases passed"