Firstly, disk defragmenters all need to have clean disks to work on. If a problem is detected with your HD, you will always be asked to run chkdsk first. In fact, as a matter of policy, I always run chkdsk before defragging - just to be sure.
If your PC has recently crashed, or failed to close properly, the PC should, automatically run chkdsk on reboot. Do not, if ofered the opportunity, interrupt this process. If you are not aware of problems, I think you will find that you may have a problem with shut down.