The SULFNBK.exe file is a utility that comes with the Win98 OS that allows users to restore long file names. Thus, anyone using Win98 would find this file on their system.

Now, the thing that set this hoax apart from others, is the Magistr virus (
http://antivirus.about.com/library/v.../blmagistr.htm ). This virus can pick .exe files at random in your Win system (they have to be smaller that 132K in length), infects them and sends them via email to other users registered in the Microsoft Outlook's address book.

That means that if you have this file on your PC, chances are that it is a legitimate utility installed used your OS. But if you received by email, it could be a virus infected file.