Good question. I have experienced this same thing and run my own program right way - Webroot Antivirus with Antispyware.
Our administrator has this to say about the problem, " We have scripts disabled on the forums, so people can't inject scripts to cause hijack your browser.... and our systems are scanned vigorously. But that doesn't protect you from rogue links and once you click on them, you will get malware popups until you clean your PC."
SuperAntiSpyware is another program is a free version you can run. Here is the link:
http://www.superantispyware.com/
Thank your for bringing this topic to the forum.
Kindly,
Kitt