Disclaimer: This should work in an NT/2K FAT-based file system.  Good luck with NTFS...  Check out 
SysInternals 
After a little security "experiment" yesterday, I messed up the Administrator accounts on 3 of my Win2k boxes.  I also fouled things up so that the other accounts on each box couldn't login as well.  Don't ask.
Anyhow, after some looking I found a few ways to fix this 
without reinstalling or otherwise messing up the OS any further.  One way involves the infamous logon.scr trick, but the fastest, best way I found is below.  
Use a boot disk (one that has attrib.exe on it), and startup in DOS mode.  If you don't have any, boot disks can be found here: 
Boot Disks.
Browse to the \winnt\system32\config\sam dir.  (If you can't find it, type "dir /s sam" in your Windows folder to find out where to go).  Type "attrib sam*.* -r -h -s" (no quotes), then type "ren sam !sam" & "ren sam.log !sam.log" -- reboot.
When WinNT/2K's login comes up, the only valid accounts will be guest and Administrator (no password).  Logon as Administrator and recreate your accounts...  That's all.  
