Windows XP comes with a piece of software, Windows Messenger (This is different than MSN Messenger) This software is, by default always running. It is a program that allows people who are on a network to send messages to each other. Which is great if you are on a LAN, but most of us home users are not. Us home users, especially those not behind a firewall, are vulnerable to a form of spam/mass advertising. A program is available that allows advertisers to send ads to a whole range of IP addresses, which means that anybody who is on the internet (even with dynamic IP addresses) can be spammed.
If you are tired of these messages and want to get rid of these messages follow the instructions located here.
First, you might consider finding out if you must be using Windows Messenger. If your LAN runs Novell, most likely, Novell has its own Send Message program, which would not be affected by spam. Consider switching over to this Novell program. If you have no choice, than you can download this system tray program which will allow you to click the Windows Messenger on and off quickly, so it is only on when you need it.