window installer window at startup

When I try to start the computer,a Dell 4600 pentium 4, a small window pops up which says windows installer.Nothing else on the desktop will work I went to taskman and it shows it running but will not go away if you click end task.
The computer had been running slow,so just before this problem started I installed and ran Ad-Aware and Window washer.The computer then ran fine,but the problem started when I tried to switch user.Now it won't work under either name. Any help will be appreciated.Thanks,Tommy

 

with it this bad I would format the drive and start over