We use Sophos Enterprise. Only drawback is that they ONLY do corporate installs. No single versions.

Pro's:
Very quick
Enterprise instalation
Centrally managed auto-updates.
Good tools for checking status of every machine on the network
Runs on EVERYTHING, including Linux, BSD, All versions of windows, etc.

Cons:
When an install fails, it's a PITA to fix (but happens rarely)
Corporate only. (although they allow any device that connects to your network to run it)

Oh, and for spam, we use a Barracuda Spam Firewall. It blocks 90% of mail entering before it even hits the email server.