You most likely have an SATA hard drive in your system.
(Make sure you know ALL system drivers you are using before you downgrade. This Might help you in finding XP replacements after XP is installed, though the HD Audio standard which combines the platform for audio and modem is still showing itself to be a big pain.)
Back to your immediate problem, You need to slip stream the SATA drivers into a Windows install CD, OR get yourself a USB Floppy and load them on from there during the XP install. I found the first option to be the cleanest and pretty easy.
I downloded the SATA drivers from the Intel website, then found this nugget called Nlite. I read the instructions for that and downloaded it for free, it used my XP install disk as a baseline. Burned a successful XP install CD on my first try.
It loaded the SATA drivers and I was good to go, until I found out, nothing else works. AHAHAHH! Thanks Microsoft...I'll get this thing running yet though thanks to the internet people. I'm online now anyway
If you contact Compaq, they will tell you that XP doesn't support SATA drives. Its a flat out lie! But they fear-mongered me into delaying my downgrade for 2 years! Until I mentioned it to a friend who said he was running his desktop with XP and SATA. So I started to dig around.