A firewall is not the same thing as an antivirus scanner. Are you sure that's the only McAfee component you are running is the firewall? If not, I would remove Avast.
http://www.worldstart.com/more-than-...s-good-or-bad/
That is a great article explaining why it is bad.
Basically, an antivirus program is like a vaccination. It uses a live virus to catch and eliminate other viruses. If you have more than one program, they attack each other, so-to-speak. And having 2 AV programs running in the background, at the same time, could cause a major slowdown of your machine.