Do not delete that file. SULFNBK.exe is used to determine long file names in Windows OS. This email was going around as a Hoax, and then someone really did write a virus for this file.

First...Go to Start - Find Files of Folders -make sure the "look in" dialoge box is pointed at the C:\ drive only. Then type the file SULFNBK.EXE.

The file should reside in the C:\Windows|Command folder. If you have other copies of this file in other folders, delete those, but DO NOT touch the file that is in the C:\Windows\Command folder.

Hope this helps...here is a article you can read regarding this hoax/virus.

Best thing to do about hoaxes is delete the email and don't send anything pertaining to it.