Defragmentation XP 
 

1 First, close all other programs and turn off your screen saver.
Close all programs that run automatically in the background, as an example; windows messenger, real player, virus software and etc. These will often interrupt the defragging process.

2 Click on Start then click on my My Computer.

3 Right Click on the C: Drive and choose Properties.

4 Choose the Tools tab.

5 Select the Defragmentation Button.

6 Click the Defragment button.

Defragmentation Vista

By default Vista is set to defrag automatically. If you right click on your hard drive and then go to properties and into Defragment now... There should be an option there that tells you whether auto defrag is on, how often it is set to run and when the last time it ran was.
It's probably easier just to let Vista automatically.