Have you done a disk check to make sure there isn't a problem on the HDD? In explorer, with "FolderView" on, right click on the "C:" drive, and then "Properties". On the "Tools" tab, click on "CheckNow" under "Error Checking" and then tick the "Automatically fix file system errors" box, then "start" - it will complain about open files, and ask if you want to schedule a check on reboot - click on "Yes" and reboot. You should get an error report, which you can check in "Event Viewer" form "Control Panel"-"Administrative Tools" in the application log.
The other thing to check using a similar technique to view C: properties is that write permissions to the root haven't been switched off.