Not alot of people actually installed antivirus software on their SharePoint 2007 environments. The unfortunate thing is, only Microsoft had something that would actually plug in (as far as I know anyway and I did a lot of research). The only solution has been ForeFront security (NOTE: this is NOT the same thing as ForeFront Identity). If you are on SharePoint 2007, you should know that they released SP3 of ForeFront security a few months ago, I just now found out which really shows you how important it is in the community (ie, not very). You should download and install the new SP3!
One piece of software that you should start out testing and considering for your 2010 environment is the Microsoft Forefront Protection 2010 for SharePoint Beta 2. It is also available for download from here.