i am assuming that u install a lot of stuff.....but u said u cleaned it.
i am guessing of the work of spyware.
newly installed xp will not be like that, unlesssssssss, u have installed each and every app that ur mobo driver provided, and also a few anti virus running at back ground with those aditional personal firewall, and some other cleaning utilities installed to ur pc.
but, good news is 500 MB of ram is enough for every thing. try install the folding@home for a change. it take up ur processor. but not ur ram. Gave me a secure feeling that no spy ware is running at the back some how. haha. since its using up my processes.
Good luck!