If you mean XP Mode, that needs 2 GB RAM minimum including what 7 requires and it only works on Professional, Enterprise, or Ultimate editions. Microsoft describes XP Mode as being for businesses needing backward compatibility, not for gamers.
"XP Mode was primarily designed to help businesses move from Windows XP to Windows 7. It isn't optimized for graphic-intensive programs such as 3D games"
Also worth knowing:
"XP Mode follows the same support lifecycle as Windows XP—extended support will end April 8, 2014."
Anyway, we're getting off track- I have an XP license. Here's a refresher
on my current situation.
Some parts of the page say it was last updated in 2009, others in 2005. In either case, I'd check a more up-to-date source; a free version of Windows XP has released as part of the Windows 7 package.