you're screwed.
Bad sector on your hard drive, time to get a new one before this one fails completely.
open up a command prompt
enter:
chkdsk /F /R
you'll get
Chkdsk cannot run because the volume is in use by another
process. Would you like to schedule this volume to be
checked the next time the system restarts? (Y/N)
answer yes and reboot and this may mark the sectors as bad so windows won't use them anymore.