Joe,
if you have an XP install CD it's easy to do what's called a clean install.
Please read all the way through before following instructions, there are one or two gotchas here!
Howto trigger a clean install:
Boot up the PC with the XP install CD in the CD drive.
When the screen prompt asks if you want to boot from CD depress any keyboard key. The install will search for existing XP installs, when it finds yours select the option to DELETE the existing partition, then build a new partition in the unused space. XP will then ask how to format the drive, quick ntfs should be fine. After that, it's a clean install and you'll be prompted to set time, date location etc.
Gotcha 1) have you an installable copy of a driver for your network interface? Make sure that it's stored on something like a USB memory stick, as you will need to re-install this later. Also make sure you have recorded any security settings for any wireless network connection required.
Gotcha 2) do you have a Serial ATA harddrive? Windows XP install doesn't always include drivers for SATA drives. The simplist thing is to set the SATA drive to COMPATIBILITY mode from within the BIOS, but I would recommend that you get copies of the driver(s) before the reformat & install when the XP install asks for any special drivers required, early in the bootup stage. For example, I have a laptop with a SATA harddrive. If I boot from the XP install CD it doesn't recognise the HDD at all! Scary if you aren't expecting it!
Gotcha3) do you have any unbranded hardware? A regular problem with home-grown builds (and also PC's with cheap/generic peripherals) Is that the hardware wizard is unable to recognise specific pieces of hardware. You can normally overcome this manually, but it really helps if you have hardware info recorded somewhere. Specifically: manufacturer, model no., serial no. If you haven't got this information to hand, before the re-format, record any hardware that has been added to the PC since you first got it. Use the Hardware Device Manager list (start - control panel - performance & maintenance - system OR start - control panel - system) to find out what XP thinks you have now, and record it somewhere.
Hope this helps.
Adrian Stone
June 2008