The way I do it is as follows:
Make the 1st boot device the CD-ROM. To do this, restart your computer and hit 'Delete' (usually) to enter your BIOS settings. Most likely your motherboard and BIOS version are different to mine, so just hunt around for 'Boot' and set your 1st boot device to the CD-ROM (in mine it's an ATAPI). If you're unsure at any stage, exit
without saving and try again.
Making sure your Windows XP CD is in the CD-ROM drive, restart the computer and let the computer boot from it. You should then follow the instructions to delete all current partitions (most likely just one), format the hard drive and reinstall a fresh copy of Windows. If you already had XP running on your system and there were no driver/hardware conflicts, you should be just fine.
