Using Disk Defragmenter allows your hard disk to be brought fully back in shape after the data deletion that has taken place with the Disk Cleanup tool and any program removal you might have performed.

click to enlarge
When Windows XP saves data on your hard disk drive, it does so in the most effiecient manner there is - it saves it in the next free available sectors. This can cause problems with performance as data can end up being placed all across the hard disk drive when in reality it needs to be stored together.
Disk Defragmenter resolves this problem, reorganising the data across your hard disk to make it more efficient and speed up Windows XP. It takes advantage of all of the now empty areas freed up by Disk Cleanup and uninstalling applications improve both the programs that are still installed and Windows XP itself.
To defragment your hard disk drive, first of all exit all Windows programs, then go to Start > Programs > Accessories > Disk Defragmenter.
Next, select the disk to be defragmented, and select the Analyze button in the lower left corner - this will confirm that the drive needs to be defragged. The process can take some time, and is best left as an overnight task for larger disk drives.