I once transferred material on CD from the one machine I had running AVG to one one of the other two both running PC-cillin (came with the motherboards) and found two infections that AVG had not picked up. No prizes for guessing which AV is now on all machines.
I think I've got to go with Mango Grummit on the anti virus line. I first got it from the Halifax BS for online banking and found it's the first one I've used in over five years that hasn't caused me a system problem. It just doesn't seem to clash with anything, unlike Norton.
It is definitely worth looking at the extra tools included in the current version of Spybot. There might be something not covered by your other programs. It showed me, for example, that I had a problem in my communications setup that I was not aware of.