A one-stop shop for information concerning games that run on Windows - that's what the Windows XP Game Advisor seems to be. From one web-based interface, you can search across hundreds of games and find out all about them - complete with demos, screenshot, trailers, etc. There's even a feature that allows you to check for updates to the most popular games. You can search by game title, age group and game type. When you've found the game you want, you can order directly from this site.
Don't know whether your puny little rig can run Doom 3? That's where the neat part comes in - by clicking on the "Will it run on my computer?" link, the Game Advisor will do an analysis of your machines CPU, memory, graphics card, disk space and operating system and tell you where your setup fits on the spectrum between "Minimum" and "Reccomended".
http://www.microsoft.com/windowsxp/using/games/gameadvisor/default.mspx