1. Insert the Windows XP disc.
2. Reboot the computer.
3. When the POST screen asks you to press a key to boot from disc, press a key.
4. An ASCII-based setup screen will appear, and the system will start loading drivers from the Windows XP disc. When this completes, you will be asked what you want to do with the installation (where you want it to go). I suggest deleting all partitions until you have a single entry that says "Unpartitioned space." This will wipe out the partition containing SBS 2003 and any other data on the drive.
5. I suggest one of two things at this point. Either just press Enter to use all of the space, or create a partition that is like 90% of the size of your disk space and leave the rest unpartitioned for a possible future use. Totally up to you.
6. Windows setup will proceed. Follow the prompts.
NOTE - If you have a RAID, SCSI, or SATA card, you may have to install drivers before Windows Setup will discover your hard drives. If so, make sure you have a floppy disk handy that contains the drivers before you attempt this. Then you press F6 near the beginning of Windows Setup to install the drivers.
NOTE - If your Windows XP disc is pre-SP2, then before you proceed with the above steps, download and burn the IT professionals installer for SP2 onto a disc and disconnect your Ethernet cable from your PC. Install SP2 prior to connection the Ethernet cable again. If you are behind a router that contains a firewall, then this risk is mitigated, though I still suggest installing SP2 while not connected to any network.
NOTE - You can use a program called nLite to create a Windows XP disc that already has SP2 installed (which will save not only time, but reduce the risk from network worms). You can also slipstream XP hotfixes, pre-install drivers, remove Windows features like Tour and MSN Explorer (plus much, much more), pre-patch Windows XP to accept third-party themes, setup an unattended installation so that all the questions the installer asks are pre-answered, etc. If you use this method to pre-install RAID/SCSI/SATA drivers, I still suggest having a floppy disk containing the drivers as a backup.