Did you prepare properly? You can't leave out ANY steps at all or you will have problems.
You have to be completely prepared for SP2 which means. Spyware free, Adware free, virus free BEFORE you install SP2, Also go to this link from Microsoft which also has links inside of it that will tell you what to do to prepare, including programs that are known to have problems which you may have to patch BEFORE you install SP2. Also go to your computer manufacturer's web site to see if any programs that came with your computer (non Microsoft programs) that need some sort of a patch BEFORE you install SP2. After all of the above, you should have no problems. Here is the Microsoft link. Hope this helps.
I tried a number of times to install XP SP2. Each time after going through the entire install process and nearing the end I get a message"access denied". Then I have to go through the process of uninstalling. I have contacted Microsoft and my ISP, the reply I get does not solve the problem.