Under My Computer, right click on the C volume, click the tools tab. In the error checking tab, click the "check now" button. Then, check the "Automatically fix file system errors check box. It will prompt you to schedule the error checking upon the next reboot as the drive is locked. During the next reboot, you'll see the error check running during boot.
If you still get the error during the defrag, select the second box "Scan for and attempt recovery of bad sectors" box under the error checking options box, reboot again to let it check, then try again.
The reason I wouldn't check the attempt recovery of bad sectors during the first run is that it takes longer, but you could check it the first time to do a comprehensive check.