If you had Vista Ultimate, you might have played the game Tinker, which is an addictive little puzzle game that has you helping a robot on a steampunk-themed stand find his way out of progressively difficult mazes. The Vista Ultimate-only game is now available as a free download for all versions of Windows from XP to Windows 7. The game has Windows Live built in so you can chat with your friends from within the game. Install the Games for Windows Live client and sign in with your Windows Live ID (connected to a Gamertag) to play.
(Tip: demo video available at Labnol.org)