To get rid of Avast then search for the uninstaller tool from their site and follow instruction. I'm of the same opinion as Tullie and prefer Avast.
Win 7 has it's own firewall, which I use, and Win Defender is an antimalware program not a firewall.
I run MSE and Windows own fire wall on W7 laptop and a Vista Desk top they work fine. MSE updates sometimes can update 2-3 times a day. I have run other free programmes including avast and they sometimes either lock up occasionally or take time booting since runnining these no problem.