Boot to the XP install disk and at the first repair console perform a repair to the system boot files. Once in the first repair console type FIXBOOT in the command prompt and press enter. Reboot the system.
Hmmm, I had that exact error due to an XP system crashing so often. I did a manual repair of the registry on that machine, but I thought I remembered getting back to desktop to do the manual repair by repairing the boot sector first. Maybe not, but that 's if I remember correctly. I eventuall reloaded that computer and found the reason for all the crashing leading to registry corruption. A bad SATA cable. Fixed and running today.
EDIT: With this particular error, a repair of the corrupted boot sector will get you back to desktop, but...it will not permanently fix the problem.
October 25, 2009 10:06:34 AM
I've fixed this error many times by slaving the hard drive in another computer and restoring the backup copies of DEFAULT, SAM, SECURITY, SOFTWARE & SYSTEM from System Volume Information. I just start with the newest and work my way backwards until the system boots. A corrupted registry can be caused by a failing hard drive or bad cable.