Is your computer part of a Botnet? The chances are very high that it has been a part of a Botnet at one time or another.

To avoid this scary nightmare, I strongly suggest you install and maintain the following preventative measures:
1. Anti-virus software
2. At least one additional Anti-Spyware, Adware, and Malware software, like:
Spybot Search & Destroy: http://www.safer-networking.org
and/or Ad-Aware by LavaSoft (there's a free version)
http://www.lavasoft.com
3. Make sure the Anti-virus firewall is turned on.
4. Do not open any emails from unknown sources.
5. Approach with caution when a friend or family member "replies all" and a zillion email addresses are always in their emails.
6. Do not download any software or any type of file, from unknown sources.
7. Critical Tip: If you want to run a clean check of your computer, you need to run all scans in safe-mode (on a PC). If you do not, you're probably not going to get rid of all issues. This is because the "bad guys" are in resident memory, and cannot be "killed" if they are running.

When you run scans in safe-mode, the "bad guys" are not loaded.

Safe-mode Instructions:
http://www.computerhope.com/issues/chsafe.htm