XBill is an open source computer game that was very popular in the open source movement at the end of 1990. The aim of the game is to Bill, a little man with glasses, and its clones, keep them to install the computer virus Wingdows on a variety of computer systems on which is already running another operating system. Wingdows here is a virus, " which has been cleverly designed to to fool the user with a popular operating system " and thus represents a parody of Microsoft Windows dar.

The various operating systems (various distributions of Linux, BSD, Solaris, SGI IRIX, Apple Macintosh, OS / 2, Palm OS, GNU Hurd and NeXTStep ) are represented by the respective logos on computer screens. If Bill has ( or one of its clones) Wingdows installed, it replaces the symbol of the original operating system by the Wingdows logo ( which, of course, the logo of Microsoft Windows ), and carries away the old symbol.

Bill can be stopped by being "beaten" by clicking with the mouse. The replaced operating systems must be pulled back to the computer with the mouse. In later levels computers are additionally connected by network cable, which Wingdows spreads faster. Some computers can thereby also catch fire and must be cleared by a bucket of water being pulled on them.