Get the latest, bug-fixed version (currently v1.2.0) from here!

This one is recommended:

This is recommended, installs both .NET and XNA required by the game.
Take this / Windows

If you already have .NET and XNA installed you can use the no-setup version:
Take this / Windows

Ratsack is also available on the Windows Phone 7:

Take this / Windows Phone 7 if you want to use the Application Deployment Tool
Take this / Windows Phone 7 if you want to use the "XNA Creators Club Game"-functionality
Take this / Windows Phone 7 if you want to (somehow?) install manually