Realistically the best anti-virus is yourself, but system center endpoint protection should be fine.
This. I've not run an AV in probably 10 years without any virus problems. Not exactly a solution for a business, but it needs to be said that no AV is fool proof.
Even then, if one were to get a bad infestation, the best way to fix it (IMO) is to run a live Linux distro from a USB thumb drive. That way, you can scan the Windows drive(s) unmounted and catch things that may be hiding in the boot sector or any other low level areas of the hard drive. When my credit union merged out another a few years back, I took a pocketful of thumb drives with me and scanned all of the computers at the other office. There was quite a bit the installed Windows AV software missed.
Not to go totally OT, but what distro/av software did you use to do this? I'm not a linux guy so this may take a little more effort for me to get this setup, but I've always wanted to make one of said linux thumb drives for such occasions.