|Actually such a message is more likely to be a corrupted registry, possibly caused by a virus, possibly not. Reformatting and re-installation is a last resort when all else has failed.|
If you can boot into safe mode do that and attempt to restore the system to a previous date via system restore. If that succeeds, turn off system restore and do a virus scan. There are many good ones available then should root out a virus. When that is completed run Crap Cleaner will should get rid any residue. Then re-enable system restore. System restore is turned of because if you do have a virus in the restore files then this will delete them.
If you cannot boot into Safe Mode then a Repair install is the next option. This will reinstall the operating system while keeping all your files and settings. You need an XP CD to do that. Once the repair install is complete then run a virus scan to make sue the system is clean, After doing a repair install you will need to re-install any updates subsequent to the installation disk that you have - SP£ at least.