I can help you out here too: it ain't going to run on the WinXP 64-bit edition. Valve hasn't added the code into the game. The only game I can think of with a 64-bit edition pending is Unreal Tournament 2004, and that isn't coming out until the final XP 64-bit edition comes out, which isn't happening until sometime next year.
Valve's already started on episodic content for HL2, so maybe we'll see a 64-bit edition available over Steam sometime.
Well crap....guess I'm gonna have to slice up the drives and put regular XP on....the things we do for games (who am I kidding? hehehe)