SAM is the Security Accounts Manager.
This could be bad. I have made attempts to recover a lost registry hive and haven't always been successful. There's a Kelly's Korner fix
for registry hives, but I don't know if it will work in this case. I used one from another site in the past but I haven't been able to locate it. This involves starting XP's Repair Console and replacing the hives from the hidden XP Repair folder. Then your computer will start like it did when you booted it the first time. Once started, you have to get into the locked system restore folders using a trick, and copy a restore point that's not today's. Then you go back into the repair console again and put that restore point in and your system should restore to what it was. Quite difficult.
Alternately, you could put the hard drive into another computer, copy all the data from it onto another drive (But you have a LOT of data!), and put it back into your machine and re-install Windows. Remember to switch jumpers from Master to Slave and back if you don't have a SATA drive.
If you have a large thumb drive, you might be able to move some of the stuff using The Ultimate Boot CD, but if your ISO's are too large, that would not be practical.
Have you tried hitting F8 right after the beep and selecting "Last Known Good Configuration?"