Try running a Repair Install. Boot off your XP CD. Choose the first option
to setup Windows XP now press Enter.
After the License Agreement there should be a time when XP tells you it is
looking for previously installed copies of Windows. It should find your
current install and offer to try and repair it. Press 'R'. Let XP repair
itself. Basically it will install the system files overtop of themselves but
leave your data intact. When finished remove the CD and reboot. The HAL -
Hardware Abstraction Layer will be rebuilt in the process. The downside is
that you will have to reinstall the multitude of XP Hotfixes But you should
be able to run your system.