I have had Avast installed for years and have never had any problems with it.
Considering you also had problems with AVG and Msgtag I would respectfully suggest that you dump the Msgtag program.
I registered with Msgtag some months ago and couldn't get on with it and so uninstalled it.
Is this the program which let's you know when mail has been looked at?? I personally didn't think it was worth the bother.