Last place used MS Forefront, got next to no viruses, current place uses McAffee and a fair bit of malware and Virus get through.
Anecdotal, I know, but I would give MS a try, it is really lightweight (resource wise) compared to McAffee and quick easy updates.
ePO is very useful from an admin point of view, but the actual client and agent on the workstations are a bit crap imo.
rp2000