Are these computers protected behind a router/firewall?
If not... Most likely the ads are coming through the "windows messenger service". (a system designed for network administrators to transmit system outtage messages and other info internally within a company) The spammers have exploited this feature to extend through the internet, and found a way to hide their true identity.
The spammers can access unprotected computer through ports 135, 137, and 139, which results in a popup message on the screen. These type of messages will appear in a little gray window that has an OK button.
This has nothing to do with MSN Messenger, or any instant message program.
Most pop up blockers only check for web based ads, which is why these are still getting through.
If you have XP, you can test to see if you're vulnerable by going to START -> RUN -> Type CMD and hit OK. In the MS-DOS window that pops up, type NET SEND 127.0.0.1 GRUMBLE! and hit enter. If you get a message that says "the message alias could not be found", you're safe. If the ad came up, you can disable this in XP by going to START -> Control Panel -> Performance and Maintenance -> Administrative Tools -> Services -> Messenger -> Start up type -> Disable -> stop.
There are some freeware programs out there that will do this for you, too. Shoot the messenger, etc Ad-aware has a plug-in to fix this as well at
http://www.lavahelp.com/articles/v6/03/05/0302.html
Here's a how-to article from Microsoft, too.
http://support.microsoft.com/default.aspx?scid=kb;en-us;330904 This problem will happen for Windows XP, 2000, and 2003 users.
Sorry for the long post. I hope this is helpful, and covers your specific problem.
Mike, resident computer tech. geek?
[ 01-29-2004, 10:36 AM: Message edited by: Mike-L@GTP ]