It's not XP Pro that's hogging, most likely you have bloatware on your PC, like Symantec/Norton, McAfee, and/or AOL.
XP Pro runs best with 512MB memory or more.
From the 512mb you have, that is plenty free. Don't forget you have virtual memory as well.
Bitdefender is not considered bloatware (but it is also not considered to be all that good either).
Maybe you should get CrapCleaner from http://www.ccleaner.com/ to rid you of junk.
Then do a full chkdsk /f and a defrag.