Computer Hangs On When Shutting Down

I have a problem when shutting down the computer. About 4 out of 5 times when I select shutdown, the computer hangs on the blue screen and I must force it to shut down by holding the startup button. Then on the next startup, about half the time, the system goes through a check.I am using XP2. I was told by Circuit City that this could be caused by spyware. I brought it to them for a fix, among other problem I have Spy Sweeper installed but even after a scan, the problem persists. Then recommended a clean install of XP2 but I am trying to avoid that route. One thing I do notice is that I have a constant correction on the startup scan with many files in a Temp folder of Spy Sweeper that are invalid. The system corrects itself, but then it happens again. Just to note, this shutdown problem was occurring before Spy Sweeper was installed.