SULFNBK.EXE

SULFNBK.EXE is an internal component of the Microsoft Windows operating system for restoring long file names.

Email hoax

The component became famous in the early 2000s as the subject of an e-mail hoax. The hoax claimed that SULFNBK.EXE was a virus, and contained instructions to locate and delete the file. While the instructions worked, they were needless and can cause disruptions, as SULFNBK.EXE is not a virus, but instead an operating system component.
Even people who didn't receive the e-mail were still perplexed if they found the file by themselves, thinking that it could be a virus or trojan horse.
In Windows 98 and ME, where the file existed, SULFNBK.EXE can be found in Windows "COMMAND" directory.
A very similar hoax happened with jdbgmgr.exe.