Defragmentation is a process that will allow your computer to open programmes and documents faster by locating fragments and putting them together.
Performing a defrag will allow your computer to open programmes and documents faster as it will not need to jump around to find all of the parts it requires.
This tip was part of an interesting IT talk I recently attended. I have certainly noticed a difference to my computer since performing a defrag.
If you want to perform a defragmentation on your computer, go to:
Start > All Programs > Accessories > System Tools > Disk Defragmenter