This is very important to do.

Disk Defragmenter consolidates  fragmented files and folders on your computer's hard disk, so that each occupies a single, contiguous space on the volume. As a result, your system can gain access to your files and folders and save new ones more efficiently. By consolidating your files and folders, Disk Defragmenter also consolidates the volume's free space, making it less likely that new files will be fragmented.

• To open Disk Defragmenter, click Start, point to All Programs, point to Accessories, point to System Tools, and then click Disk Defragmenter
• When it opens click analyze. When that is done click defragment.
• If you have any question just ask.