McAfee and Norton are my last choice. They are resource hogs and expensive. I work for a software development company and we install Microsoft Security Essentials on all of the computers we install. If you are having problems with viruses, then likely someBODY at your office is the problem...going to sites that are virus-prone or clicking on links. If we have clients that have repeated virus problems, we install
openDNS to restrict them from virus-prone websites.
The aforementioned tools are good for ridding yourself of the virus already present. If it were my home computer I would probably do a clean install of Windows after formatting the drive (back up your data of course).