Hi, as Placeholder said do you have another account? if so boot to it, if not try safe mode, if this does not work, post back we will need to access a CMD prompt and use the default Admin account.
This is a bit involved and there are methods to fix this if you Google, problem is if you get it wrong then Grief. So no BRINK solutions people please. We will need to see the relevant Registry keys for the profile. Do you have your copy of Vista's DVD. If not, what make model computer do you have. We need to have you post the contents of this reg file:-
regedit /e C:\Note.txt "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList"
This command when copied into the run command will give a Note.txt file in the root drive C and can be copied and pasted into here. We need to see if a shutdown has corrupted the profile OR something else (looking for .bat or .bad after the SID1004 or 500 entry)
Problem is someone else may have to help you I have a few legal problems at the moment (accident, not criminal yet very costly).