If these computers are not on the internet where are you expecting to pick up any virus infection?
Why pay for antivirus when very good protection can be had for nothing. Try Grisoft AVG7.Which is free.
Mcafee is a paid for pain in the butt.As you have already found they are not the most helpfull company.
I really dont understand what your problem is if you are not connected to a likely source of virus infection then you should not pick up any nasties.