Cheers Munkery,
Is there a plug-in you know of for Firefox?
Yeah I downloaded ClamXav today, if it's as good as ESET then I will install it, plus it's free so that is always a plus!
I believe "No-Script" provides that type of functionality for Firefox.
ClamXav should be sufficient for your needs. ESET, and other AV software, that run with elevated privileges can actually introduce remote and/or local privilege escalation exploits into your system. For example:
http://www.exploit-db.com/exploits/3386/
http://www.exploit-db.com/exploits/14818/
ClamXav does not have any components that run with elevated privileges unless you use the "Sentry" feature. Even if you use the Sentry, it is not the part of the AV software that receives updates or performs scans. The Sentry just watches user specified folders for changes to initiate scans by other unprivileged components.
The only other AV software for Mac that does not run with elevated privileges is VirusBarrier Express (Snow Leopard only) from the Mac App Store.