|Yeah the OS won't work but the keyboard ought to. You should at least be able to enter bios setup the same way as before. If you're using a USB keyboard and have PS/2 ports use a PS/2 keyboard.|
If that doesn't help, disconnect the hard drive you had previously removed and reinstalled and try booting up. See if you can get into bios setup.
It's possible that whatever motivated you to reformat and install xp was a hardware and not a software problem. Or you may have jacked something up while inside the case.