Scan Disc helps your computer to run faster.  You should perform this operation at least once every 3 - 4 months.

To run your Disc Defragmenter it is best to run scan disc first, to rid your PC of any errors.  The errors will prevent the Defragmenter from completing it's task.

Once there are no errors on the PC, make sure that you run the Disc Defragmenter program in "Safe Mode". 

To use Disc Defragmenter, run in safe mode for best results, restart your computer, and press F8 as it begins to startup (while the screen is black) until you see options for startup appear.  Choose Safe Mode.

To begin, click Start>Program Files>System Tools>Disc Defrag.