Like Andy said, your system cannot have a memory leak...that's basically impossible. A memory leak is the result of sloppy, or somehow "wrong" coding in some type of software / program / game. When running a program that has such a "bug" if you want to call it that...it just keeps using up more and more system resources / Ram until it crowds out, or "possibly" even over-writes the area your Operating system needs to function...then you crash, lock up, BSOD or some similar situation happens.
Reformatting is "always" good, if you have everything backed up, and can spend the time doing so. Make sure you have your mobo ( motherboard ) drivers CD to reinstall all of those and if not...download them from your MOBO's manufacturer's website.
Try using these free programs before doing so...they might pick up some stuff that's slowing your system down.
http://www.filehippo.com/download_ccleaner/I've used the above program now for over 3 years...no ill effects. Make sure, the link your downloading from is the on the "right" over in the green area where it says "Download latest Version" under the green arrow. I once told my Dad to get this program and he tried to download off a link to some "pay" program on the left side. He tried 4 times and later told me that this program wasn't free LOL
http://www.emsisoft.com/en/software/free/I find spybot search & destroy isn't up to snuff as it once was...the above program updates almost daily with a new malware / addware / trojan database for the latest threats out there. For a free program it's a pretty good spyware removal tool.
Stop using Internet Destroyer ( ahem...IE ) and use Firefox, or another open source browser...you'll save yourself a TON of grief...to get it go here...
http://www.mozilla.com/en-US/firefox/When your McAffee subscription runs out...if you switch over to Firefox, you can get by easily with either of these 2 free AV programs...and they use much less system resources than your current AV program. I happen to like Avast ( now that it's up to 4.8 ) better...but both are good. Make sure to uninstall McAffee...
http://www.avast.com/ Look for the free home edition !
http://free.grisoft.com/ Again...it's free
There are better AV's out there, but you have to pay for them...and most are resource hungry and bog your system down. These 2 are pretty light on resources...you can even use them BOTH together, even though it's not recommended.
Lastly...please get yourself some MORE ram...512 ram for any gaming is like basement level minimum...and RAM is dang cheap...go here...
http://www.newegg.com/Index.aspx?Name=Home to get just about any thing you might need
Once you get your ram ( however much ) and install it, manually set your page file to zero...restart your computer...then reset the page file to double the amount of ram you have installed and reboot once again.
If you have 1 gig of ram set the page file to 2 gigs...2 gigs of ram...set it to 4 gigs...that would be 4000 Megs. Your page file will now stay the same, never defrag...and Windows won't waste it's time resizing it all the time, which usually improves your gaming perfomance...usually.
There are more things I could tell you, but they involve messing around / tweaking the XP OS...try the stuff above and see what your results are