The lightest of the four are Avira and Avast, so if Avast works for you there is no reason to change. I use to put my daws in the exclusion list at the antivirus' real time protection settings. Also auxhost.exe (jBridge).
All the main free antiviruses are tested daily here:
http://www.westcoastlabs.com/realTimeTe ... ID=1#item1
Today Microsoft and Avast win, but it changes every day. It's fun.
Well...moderately fun.