This is a multipartite virus which infects MBRs on hard drives and
COM and EXE files when they are executed. Occasionally the virus
tries to play something (probably laughter) from the PC speaker.
Yesmile is a stealth virus, so you can't see it in files or
boot sectors when it is resident in memory.
There are several variants of this virus. The 5504 variant was
distributed in a dropper called LAUGH.EXE in usenet newsgroups
in January 1996. Do note that F-Secure anti-virus products do
not specifically detect the LAUGH.EXE dropper: you will have to
delete it manually.
[Analysis: Mikko Hypponen, F-Secure]