Sounds like a driver issue. Reusing an old XP instillation can be tricky when moving from one motherboard to another. This can really be a problem when the chipsets used in the motherboards are quite disimilar (like the Nvidia based MB you were originally using and the AMD/ATI based MB you're now using).
When you swapped the drive to the new system did Windows load additional drivers for your new MB? You can find them in Control Panel under Hardware in System Properties. Often Windows can recogize older chipsets and install correct drivers for them, but with newer chipsets (like yours), Windows may need a driver disk. Check and see if your drivers are present.
Also, you may have some conflict with the older Nvidia drivers. You can probably deleate those while in Safe Mode (keeping in mind that this will prevent you from correctly run the drive with the older motherboard).
Finally, whenever I've done this type of upgrade in the past I've always started by copying the drive to a new hard drive and using that drive to run the new system. This prevents any changes you make to the OS from redering your original instillation unusable. Once you've made sure the new system is problem free you can wipe the old drive and use it for additional storage.
Just some things to think about.
BTW - While they frequently were looked down upon, I had the best luck doing this type of upgrade with VIA chipsets (because they all use the same Hyperion driver). Still, if you're careful you should be able to figure this out. Good luck.