windows vista suddenly running extremely slowly

Hello,
I have a duo centrino PC running windows vista. My laptop was working well up until last week when I shut down the computer without any problems. The next time I started it, I noticed it was running EXTREMELY slowly and It was taking 10 to 15 minutes to start basic programs such as internet explorer or word. I tried running McAfee figuring the computer may have acquired a virus. When I would try to run a full scan, it would give me an error saying that a file was missing and the scan could not be run. So I figured the computer had a virus. I went to safemode and the computer runs with good speed in safemode. I tried to reinstall McAfee but it can not be installed in safemode. I tried installing it in regular mode but when I click on setup file, nothing happens. Could it be that a virus is preventing me from installing McAfee? The fact that the operating speed is good in safe mode but extremely slow in regular mode be related to a virus? Would really appreciate your input on how I can get my computer back.