Why is that your first guess? Sounds like some corrupted OS files, possibly a corrupted HDD sector.
Either way, the data's (likely) still there, but the OS will either need to be repaired/reinstalled or a new HDD needs to be purchased and installed.
I highly recommend one of these for your toolkit. If the OS is irretrievably gone, then you can still take your internal HDD, plug it into this thing on another computer, and backup whatever files you can access. The computer will just read it as an external HDD. This thing has saved my life many times.
BFLR