Firstly, do you mean colourful pop-ups full of pictures and fancy writing, or have you recently upgraded to Windows XP and are you getting the plain text Windows Alert System Messages (or whatever they're called)? The former should be easy to stop - just don't visit porn sites. The latter is a touch more difficult because, for some reason, Windows XP sets the default setting for these annoying messages at "on" and they're not easy to turn off. They can be sent by anyone and are sent to any computer anywhere just by typing in the IP address (and there are programmes available which will send messages to tens of thousands of IP address per second.
To disable Windows Messenger Service pop-up ads:
1) From the Start Menu, select Settings;
2) Open the Control Panel;
3) Open Administrative Tools;
4) Open Services;
5) Double-Click the Messenger service;
6) Configure the start-up type to disabled.
This page helps:
Hope target=_blank>http://www.itc.virginia.edu/desktop/docs/messagepo
pup/
Hope this helps.