This sounds very much like your hard drive is failing.
Do you have a USB key?
If so, download HD Tune from
http://www.hdtune.com/hdtune_255.exe and save it directly onto your usb key.
When it has finished downloading, run the file and install it, once agin, directly onto your usb key.
Once the install has completed, run the program from the usb key and scan you hard drive for errors (I think the option is in the fourth tab along).
This will scan your hard drive for bad sectors.
When the scan has finished, look at the screen. If all of the little squares are green then the hard drive seems okay and it would be worth opening a command propmt and typing "chkdsk C: /r" without the quotes. The operating system will say that the drive is in use and ask if it should run at the next boot. Choose yes and restart your PC.
If any of those little squares are red then it is time to back up any important data and get the hard drive replaced before any more problems occur.