diff --git a/scripts/windows/msi-install.ps1 b/scripts/windows/msi-install.ps1
index d864f19c..9aa7ff5f 100644
--- a/scripts/windows/msi-install.ps1
+++ b/scripts/windows/msi-install.ps1
@@ -5,6 +5,25 @@ param (
     [string]$BuildDir = "."
 )
 
+# Clean up leftovers of a previous test run that did not complete its
+# uninstall (e.g. was killed, or the uninstall failed to remove files).
+# A leftover data directory makes the MSI installation fail with
+# "data directory exists and is not empty", permanently, until the
+# worker is cleaned.
+$svc = Get-Service $ServiceName -ErrorAction SilentlyContinue
+if ($svc) {
+    Write-Output "Removing leftover service $ServiceName"
+    if ($svc.Status -ne 'Stopped') {
+        Stop-Service $ServiceName -Force -ErrorAction SilentlyContinue
+        try { $svc.WaitForStatus('Stopped', '00:01:00') } catch {}
+    }
+    sc.exe delete $ServiceName | Out-Null
+}
+if (Test-Path $InstallDir) {
+    Write-Output "Removing leftover installation directory $InstallDir"
+    Remove-Item -Recurse -Force $InstallDir
+}
+
 # Look for the first MariaDB MSI file in the $BuildDir
 $msiFile = Get-ChildItem -Path $BuildDir -Filter "mariadb-*.msi" | Select-Object -First 1