If I run two anti-viral programs as suggested, won't each detect the other as being responsible for suspicious activity and as a result cause havoc with my machine?

That's a good question. I have one anti-virus program installed plus No-Script which automatically blocks malicious scripts, click-jacking and cross-site scripting. The user chooses which sites to allow and prevent from running scripts. No script is free (donations appreciated) and there are constant updates.