I had this problem and had to reintall Windows. It was a real mess.
You might try the reboot with Last Known Good Configuration Feature - did you try this?
What I had to do as nothing else worked, was to reinstall Windows in a second directory, Log into this second installation, backup all my data, make a copy of everything in the WINNT/SYSTEM32/ Directory, then reinstalled Windows over the original installation. Then I went into the second installation and moved all the files from the WINNT/SYSTEM32 backup into the reinstalled location - minus the most recent files (do a search and find the most recently modified files - these are the registry files). The I logged into the New installation and restored my registry (I had backed it up recently - if you haven't you may be out of luck - use regedit). Then I edited the registry to use the correct Documents and settings files using redit.
Over all Most everything worked OK - I had to reinstall the latest updates for WIN2000, Office XP, and Wordperfect, as well as a couple other programs. I also had to reinstall my LAN connection for my Cable Modem, and my Video and Sound Card drivers.
It took me about 2 days to get back up and running.