Sadly, what we have here is a corrupt registry.
If you're lucky, it may only be a dodgy IDE cable corrupting the HDD Signal. More likely to be either malicious Virus/spyware SW or hardware on its last legs.
Causes? Well, HDD's are tempramental, more so than most other components (lots of small moving parts you see).
Solutions?
You could perform Scandisk (O/S permitting) and mark the bad areas of the drive.
Run Regclean.
Roll back your system to "last known Good" restore point.
Worst case scenario, Bin the drive, buy a new one and restore your latest full system backup (you do have one don't you?)
Mick Rutherford, January 2005