Software problem? I would try running chkdsk to see if it finds anything. Start creen, type cmd, Right Click on Cmd prompt and 'Run as Administrator'. Type chkdsk /f /r then Enter. (note spaces before /). Type Y and Enter to run the program when you restart Windows. Expect it to take a long time if you have a large drive. Use this to check out the chkdsk log http://www.sevenforums.com/tutorials/96938-check-disk-chkdsk-read-event-viewer-log.html
If that fails to cure the problem, try Refresh Install. Start Screen, type 'Refresh' and choose 'Refresh your PC from Settings. You'll need your installation disk. Your data will be preserved but some Apps will be uninstalled