Run Check Disk

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