SAN JOSE, Calif. (AP) — Some users of the popular AOL Instant Messenger program were bombarded Wednesday with messages seemingly from friends that linked to a humorous Osama bin Laden game.

Downloading the game, however, installed a piggybacking program that broadcast the advertisement from the infected computer to all correspondents on its AIM buddy lists. The software, called Buddylinks, is not technically a virus because users must accept its terms of service before it's installed. The small-print legal disclaimer states what's being installed, though users tend to click through such legalese without reading it. And that's one of the keys to its success.

The program is also clever in its use of social engineering to spread, extending a personal invitation that appears to come from what is typically a trusted friend. "In addition to being a particularly slimy form of adware, is also a violation of the AIM terms of service," said America Online spokesman Andrew Weinstein. "We are actively investigating what legal options we have to prevent this company from doing this."