1. Remove the SATA connectors from all hard drives
2. Buy a new hard drive and connect it to the MB and power supply.
3. Boot from the XP DVD. If necessary, change the boot order in the BIOS to start with the DVD drive.
4. After the installation is complete, connect the older hard drives again.
5. Go to the BIOS again and change the boot order if you want. If you're going to boot into XP most often make the new drive first.
It's important to disconnect all HDD drives except the one where you will install XP. If you don't, there's a chance they will be corrupted. It happened to me - two drives ended up empty and showing sizes of only several MB instead of 1 TB.