Chkdsk is a standard, built-in windows tool used to find and fix disk errors
Click: Start -> Run
type: 'chkdsk c: /f ' without the quotes, and click: OK
When your asked: 'Would you like to schedule this volume to be checked the next time the system restarts? (Y/N)', choose Y
Restart your computer
NOTE: this could take a long time [ie., many minutes] to complete