Hey Bads,
My guess is that XP is using that extra ram... Instead of placing info within the pagefile, it throws all that info into ram which it can allocate... Look at all those services running in the background that XP needs to load up.
It is faster to pull things from ram then your HD... That is just my guess. And of course, I could be way off.
I hope this somewhat helps answer your question.
-SlickVic78