The semi-official Gamerscore Blog run by Microsoft employees confirmed in the article "The Sixth Tech Myth" that Live Service will be available on Windows XP.
"When Cliffy B demoed Gears of War for Windows at E3 2007 to a throng of enthralled attendees, he let drop at the end of his presentation that Gears would be available for both Windows XP and Windows Vista this year."
"You should all breathe a sigh of relief and know that Games for Windows -- LIVE will work on Windows XP."
"... the team has been able to go back and finalize the work done to make sure Games for Windows -- LIVE works on Windows XP. Which means that not only Gears of War will connect to LIVE on Windows XP, but so will the other games we announced that will support Games for Windows -- LIVE, like Universe at War from SEGA (which, incidentally, will also enable cross-platform play between Xbox 360 and Windows PCs)."