If you can boot into Safe Mode, you should be able to do some basic cleaning and Error Checking steps which might fix the issue.. It would also be nice to know if you've done any checking for malware such as antivirus and antispyware tools you've used.. Spyware, etc. can cause such issues.

How To Start In 'Safe Mode'

I would first start by cleaning up the computer using the instructions below. The steps below may take a while to perform but they should help.:

For Windows 2000/XP, ignore the first couple of sentences about checking "resources" and the "Performance tab" 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

Because the links for the "Services" are no longer functioning in the link above, use this one:

http://majorgeeks.com/page.php?id=12

AFter that's done, clean out some of the junk files using the link below. (On Windows XP, it's best to NOT delete the .chk files as indicated in the article..):

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.

Grif