PIF stands for "Program Information File", and basicly contains low-level information for launching MS-DOS files. Unlike INF files, PIFs are obsolete. I don't know of any modern program that uses them.
Actually e-mail viruses are just EXE files renamed to PIF, so the OS will treat them differently.
Unfortunately, viruses can also spoof e-mail addresses, so it almost never comes from where it says it does. I get viruses from my friends' e-mails, yet they all tell me they don't have viruses (any smart Windoze person has Norton).
