Newest update: Games for Windows - LIVE (It will be called this way) will be launched in the USA on the 8th of May. 10 days later (18th May), we - the Europeans - will have the chance to enjoy Games for Windows - LIVE. This is the same day that HALO 2 is released.
Games for Windows - LIVE has all the advantages of the XBOX Live variant. So XBOX Live gamers will be able to game with their account in Games for Windows - LIVE. As on the XBOX, Silver membership is free, Gold (extra features) is paying.
To give an idea of the features, I've found this list:
Silver Membership Features:
• Single gamertag
• Common gamer profile
• Common gamerscore
• Single player achievements
• Private chat via text and voice
• Common friends list and online presence
• PC only multiplayer including browsing a list of active PC games
Gold Membership Features:
• All Silver membership features
• Multiplayer matchmaking with friends
• TrueSkill™ matchmaking
• Multiplayer achievements
• Cross-platform gameplay
HALO 2 will be the first playable game through the new network. The first game where PC and XBOX gamers will be able to compete is Shadowrun which is due to be released later on. Next to that Microsoft has announced that Games for Windows - LIVE is ONLY compatible with Vista.