For the more confident, there are things that can be done with the recovery console (command line), assuming that it is accessible and "administrator" still has access to the local disk.
Namely - renaming the C: \Windows\system32\config\security file to (say) security.old and copying in the security file from C: \windows\repair.
Similarly for the SAM file - this is where the user passwords are stored - it's not this that has been corrupted, though.
SD