As with any computer, whether on a network or not. Id recommend at least 2 virus protectors and spyware detectors.
I use norton as my primary virus software and to be honest its the best ive used. But i also have a a copy of AVG on my pc too! From time to time i activate AVG and it finds the odd thing that Norton doesnt. (if i run AVG as my primary virus protector then run Norton as my backup, Norton finds loads of stuff AVG has missed).
Same thing with the spyware detectors. I use Spyboy as my primary software but use Spyware Doctor as my backup.
I wouldnt recommend that you use Windows XP anti-virus gear because apparently its poo.
Im not 100% but i think that you might be able to get a "Network Copy" (not sure if thats the correct term) of Norton which will allow you to install it on each machine for a fixed price rather than buying it for separate machines. - I know that my old school did that.
Hope that helps - It makes no sense to me! But hey! Im drunk! Night!