I reformat my notebook every four months to get rid of site registry checkers. The version I use is an academic Windows Professional XP OEM. To do so I put the disc in the CD drive then turn off the computer. When I turn the computer back on, the boot ROM will initialize up. Then in the top left corner it says "Press any key to boot from CD". Hitting a key before five seconds are up and it will begin to load the format utilities that install a brand new XP.
After five or so minutes, a blue screen should lead you through formatting options. I suggest deleting the entire drive. Afterwards, you'll have to reformat. That will take about 15. Then the system files will automatically install. When finished it will say rebooting in 5 seconds.
At this point you must remove the CD before the computer restarts itself or it will repeat the steps I just explained. Then restart the computer and Windows will ask for the CD again. It continues by distributing the file system into it's necessary contiguous allocation. That should take forty minutes.
This is OEM procedure only. If your using a manufacturer's system image, the technique will differ.