...scanned the computer for viruses and spyware? You can do it in Safe Mode, have you tried? Which antivirus and spyware removal tools do you have installed?

While in Safe Mode, please shut down all the unnecessary start up programs that are running in the background. Then clean out the junk files on the computer. Follow the steps in the links below:

For Windows 2000/XP, ignore the first couple of sentences about checking "resources" in the link below but follow the rest of the procedures including the "Services" links at the bottom.

How To Shut Down Unnecessary Start Up Programs

How To Clean Unwanted Files From The Hard Drive

After following all the procedures above, run Error Checking/Chkdsk and Defrag to optimize the hard drive:

How To Run 'Error Checking/Chkdsk' On Win2000/XP

Hope this helps and let us know how it goes.