Visit: http://www.gamesforwindows.com/en-US/Support/live/connect/Pages/joinliveexisting.aspx
also, heres whats available:
Battlestations™: Pacific
The Club™
Fallout® 3
Gears of War® for Windows
Halo® 2 for Windows Vista™
Kane & Lynch: Dead Men
Lost Planet™ Extreme Condition - Colonies Edition
Shadowrun™
Universe at War™: Earth Assault
Viva Piñata™ for Windows
Warhammer® 40,000™: Dawn of War® II