How much RAM do you have?
Have you tried 'System Restore' back to before it started happening?
Try the following...
Run 'CHKDSK' Start>My Computer, right-click your HDD usually C:\ and select 'Properties'. Click on the 'Tools' tab at the top of the properties box.
Under 'Error Checking' click 'Check Now' In the drop down box you'll get a choice of...
1-Automatically fix system errors (and)
2-Scan for and attempt recovery of bad sectors
You can run both if you like, it wouldn't hurt.
But do them one at a time. If you click the first one (auto Fix) it will tell you it can't do a check until you next boot. Tell it to do this then come out and reboot.