I have Norton 360 on my computer at home, and although it seems to do a good job (but then again my wife's teenage daughter doesn't even know I have wireless internet on my computer and laptop) it seems to hog CPU time at times. I can be using the computer and all of a sudden it gets noticably slower. If I look at TaskManager, Nortons is chewing up the CPU and thrashing the disk. It is, however, quite easy to use.
If I was choosing again, I might go for something else that does not get so greedy.
Problem with all this virus stuff is that eventually a scan will take a full 24 hours! So you will have to allocate one of the CPUs to do a continuous scan...it will be a sad day