one other thing.
i once tried reinstalling xp, it started so i left it to its own devices, came back all done. I soon realised though, that as it was reinstalling it, it had realised that xp was already on the hard drive, it found another partition on the hard drive and installed it self there, so in effect i had xp x 2 not good.
If you have that may problems format your hd and start again.
but to be honest i dont think you really need to, when i had win 98 i used to format the hd once a year (approx) since i have had xp i have never need to, as has been said, you can run disc clean, defrag and best of all system restore.
Its amazing what junk you do acumilate on a pc and a really good clean out / virus check / specialist fixer disc (i have norton) will do