Halo 2 Coming To The PC... With A Catch

February 09, 2006 - This is cute - an internal Microsoft team is porting Halo 2 for the PC, but you can only play the game if you purchase and move over to Vista once it's released, whenever that may be...

"Halo 2," the game that redefined first-person combat and multiplayer action for millions of gamers worldwide, is set to explode onto PCs exclusively for Windows Vista. "Halo 2" for Windows Vista will be developed by a dedicated Microsoft Game Studios team in partnership with Bungie Studios.
http://ve3d.ign.com/articles/687/687236p1.html