I am a Network Engineer/Administrator by trade. But please, take everything I say with a grain of salt as this is still my opinion, albeit a professional one.
For our Corporate network, we use Symantec AntiVirus Corporate Edition. It's far different from your standard Symantec/Norton, neither of which I could recommend for consumer use.
For any field machines, or personal machines we tend to use Microsoft Security Essentials, and are working on setting up and installing their Corporate or Business version of the software. It runs great, also helps prevent Spyware as well as Virus attacks. There is no AntiVirus software, and anti-spyware that Prevents all spyware from loading on your computer. This includes Microsoft's software, however it is the best we have seen thus far at preventing. Microsoft may not be the most liked company out there, but they are very good at what they do.
So, my personal and professional opinion would be to download Microsoft Security Essentials directly from their site.
Hope this helps.