Similar to other answers:
Make sure you have plenty of space free on your C: drive.
When defragging, you need to keep rerunning it over and over until it stops improving (with XP in normal startup mode usually this is 2 or 3 attempts).
Personally I would recommend a free app called myDefrag and choose to run the 'monthly scan'. www.mydefrag.com/
As Peccavi mentioned, there are lots of free spyware removal tools out there and it's just a matter of personal taste which one you use. I have moved on from Spybot S&D and instead use one called Super AntiSpyware www.superantispyware.com/ (seems to find more). If you go for something else then be very careful as there are a lot of fake spyware removal tools out there (which are actually infested with trojans or other malware).
I assume you have up to date antivirus? If not then I'd recommend a good free one called Avira AntiVir http://www.free-av.com/ (I find more reliable than AVG / Avast).
Lastly as others have mentioned, over time more and more apps creep into your startup list (legitimate apps and nasties). The problem is there are so many different startup locations in Windows. I'd recommend a free app called Autoruns which will actually show you everything that starts up (and lets you remove them) http://technet.microsoft.com/en-us/sysinternals/bb963902.aspx.
Be very careful removing items from starting up if you don't know exactly what it is you're removing, as there's a lot of system stuff in there you won't recognise but is still required. Prob worth doing a system restore snapshot before making any changes:http://www.microsoft.com/windowsxp/using/helpandsupport/getstarted/ballew_03may19.mspx
Best of luck
Steve I, July 2010
