diff --git a/install.ps1 b/install.ps1
index eed8919..8c8fd70 100644
--- a/install.ps1
+++ b/install.ps1
@@ -48,7 +48,10 @@ if (-not (Test-Path $CoPy)) {
   $tarball = Join-Path $CoEnv $asset
   Invoke-WebRequest -Uri $url -OutFile $tarball -UseBasicParsing
   # tarball contains a top-level "python\" directory.
-  tar -xzf $tarball -C $CoEnv
+  # GNU tar from Git Bash on PATH parses the "C:" as a remote host; pin to Windows' bsdtar.
+  $tarExe = Join-Path $env:SystemRoot 'System32\tar.exe'
+  if (-not (Test-Path $tarExe)) { $tarExe = 'tar' }
+  & $tarExe -xzf $tarball -C $CoEnv
   Remove-Item -Force $tarball
   if (-not (Test-Path $CoPy)) { Write-Fail "python bootstrap failed: $CoPy not found after extract" }
 }
diff --git a/install.sh b/install.sh
index 728c81c..88594f6 100755
--- a/install.sh
+++ b/install.sh
@@ -37,6 +37,10 @@ if [ ! -x "$CO_PY" ]; then
   case "$uname_s" in
     Darwin) TRIPLE="$ARCH-apple-darwin" ;;
     Linux)  TRIPLE="$ARCH-unknown-linux-gnu" ;;
+    MINGW*|MSYS*|CYGWIN*)
+      red "Windows detected (Git Bash/MSYS). Use the PowerShell installer instead:"
+      red '  irm https://raw.githubusercontent.com/openonion/oo/main/install.ps1 | iex'
+      exit 1 ;;
     *) red "unsupported platform: $uname_s/$uname_m"; exit 1 ;;
   esac
   ASSET="cpython-${PBS_PYTHON}+${PBS_RELEASE}-${TRIPLE}-install_only.tar.gz"