You need to be on SP3. With SP2 your system is unprotected. Whatever problems your are getting that slow down your system are unlikely to be caused by SP3, millions of people have it without errors. It is most likely due to your software or some minor hardware fault.
Please tell us what security software you have installed, anti virus, spyware and/or firewall.
Do you regularly use any kind of registry cleaner or windows defrag.
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Back to the original problem, I would now run system file checker. Click on Start and type sfc /scannow (include the gap before the /) and hit Enter. You will most likely get a message asking you to insert the XP disc.
The only hitch you may have with this is that your system is on SP2, if your OS disc is not on SP2 or higher it may not run.
This guide below will show you how to slipstream your Windows XP disc with service pack 3, you can then use that to complete the operation.
http://www.helpwithwindows.com/WindowsXP/Slipstreaming_Windows_XP_Service_Pack_3.html
It may also be worthwhile to run a disc check to check the health of your hard drive.
Click on Start then Run, type cmd in the box and hit Enter. Then at the prompt type chkdsk /r (include the gap) and hit Enter, you will then get a message asking if you wish to run this on a reboot, answer yes and reboot the PC, on restart the chkdsk facility will run, this may take up to an hour and might at times appear to have stopped, leave it to finish and observe any error messages.
