Xp Pro Box Slow After Update To Sp2

[kvarnerexpress 0]

I just reinstalled Windows XP Pro on my wife's 1.5gHz P4 256MB RAM machine. It was fine for a day, until I updated it to SP2. Now it is very slow- just right-clicking on the desktop takes 8 seconds to show the menu. Before the format, it was the same OS, also SP2. I have added a few programs, such as Open Office, BrilliantPhoto, Adaware, and others. I do not know if the problem was before or after the installation on these programs. Some of the were installed before SP2, some after.What could I troubleshoot? I am not familiar with Windows. Thank you.