Try Safe Mode first. As soon as you turn on your computer and before
the Windows Logo appears, press F8 a few times until you get the options
where you can choose to boot into Windows in Safe Mode (with Networking
is what I would suggest). Now, if the keyboard works, I would download
and install the latest chipset (motherboard drivers) for your computer.
Try using Everest Home from
www.lavalys.com to find out what
motherboard you have and it should even provide a link to the proper
drivers. If they keyboard doesn't work in Safe Mode, try the method below:
Try booting off your Windows XP CD, choose to INSTALL, then press R to
repair. Now the keyboard should be working throughout this process and
XP will repair itself. What happens here is that it overwrites the
critical system files with the ones from the CD in case any files were
modified or removed that were needed. Note this will not delete any of
your personal data, but it will remove all security/hotfix updates you
have downloaded for Windows. You will want to go to Windows Update
immediately after doing this.