It sounds like the file ntdll.dll has become corrupted. You'll need to get to the Recovery Console (I'm assuming you're running either XP or 2000 Pro) and try to rename the file or try copying the file from c:\windows\driver cache\i386. Also, you can run sfc /scannow via the Recovery Console (once you're in the Recovery Console you'll be prompted for a password to the administrator account...if you don't remember giving it one, just hit enter). Running chkdsk wouldn't hurt either.