Try starting *completely* from scratch, doing the following in the order listed:
Take your time. Be methodical. Have everything you need before you start, including a notepad and pen to write down any error messages you may receive during the installation process. Be sure to write down the error messages exactly as they appear, word for word, letter for letter - knowing the exact error message will help tremendously in solving any problems you may encounter.
(1) Get updated drivers for *all* of your hardware, if possible.
(2) Disconnect all non-essential hardware, i.e., hardware that your computer doesn't rely on to operate, such as scanners, zip drives, printers, etc.
(3) Reformat the hard drive.
(4) Reinstall XP.
(5) After reinstalling XP, before installing *anything* else, install all of the drivers for your motherboard - BUT instead of using any updated drivers, use the original drivers that came packaged with your motherboard. (If you've gotten this far, I wouldn't worry too much about installing drivers for your hard drive, as Windows has obviously recognized it.)
(6) Install any hardware drivers that are essential, and/or that Windows recognizes as needing to be installed. Do not connect or install any non-essential hardware yet.
(7) Install any other software you consider essential for the operation of your computer.
(8) Install SP1.
(9) Install the updated drivers for your motherboard.
(10) Connect the rest of your hardware and install the drivers.
(11) Install any remaining software you want on your computer.
This may or may not help, but at this point, it sure can't hurt to try.
Good luck!
Mollie