I'm also going to go with NOD32.
I really don't like Symantec. They're better than they were a few years ago, but still bloated. BitDefender has to be the worst I've ever used - it's a cow and we've had nothing but problems with it. Viper isn't too bad and is light weight, but we've found that it has let a few things through that it shouldn't.
NOD32 is the only one I've found that works good consistently, has a nice administrative interface for you as the admin (lets you push, monitor who has updated recently, etc.), and isn't a system resource hog.