When i install windows xp i done it 2 times

What exactly do you mean? Well, you can always format the drive. This will delete everything on it, and then you can install Windows and start from scratch. But if you want to keep intact all your data and files, then you have the option of defragmenting the drive.
How to defragment: Go to "Start -> All Programs ->Accessories -> System Tools -> Disk Defragmenter". You can select the drive you want to defragment and press on "Defragment" button. Defragmenting helps you free up space