My line of expertise is computer security.
When tuning up a new PC, (one I've not seen before) the first thing I do is remove McAfee or Norton or both. They are both "Bloatware" and will slow down almost any system.
Then I install AVG 7.1 FREE (anti virus) and
Trojan Hunter (30 day free trial) to scan for Trojans
Windows Defender (only after explaining to customer that its a BETA)
None of the above mentioned programs actually RUN on a PC.
They are only active while you're doing either an Update or Scan.
Except the resident portion of Defender.
It plays well with all the others including AVG.
Using this "Package" of software, I've never seen any conflicts or slow-downs with their use.
It is common practice to NOT (as in NEVER) run two AV or Firewall programs at the same time. They will not strengthen each other, but they may fight with each other or other software trying to run.
I've found that AVG (anti virus) and ZoneAlarm (firewall) work well together. You do need to shut off email scanning in ZoneAlarm.
I hope that helps some....(someone, somewhere)