I also faced same problem!
The scenario is like this ...
Our college have a nice cyber cafe. One day I used an infected pc from cyber cafe to check my Yahoo! account. From that day, my contacts started to get spam messages with Virus attachments.
So, I checked some of those mails. They were came from same machine (IP).
Next day, when I go to college, I checked the IP range 'n voila, that IP was within range.
