Windows 7 or 8? I have used IE11 with it, and all is fine.

What many clearly do not understand is that it is impossible to write one piece of software, which is going to perform without flaw on every single machine. You may have another software program on your machine, which is conflicting with Avast. One thing that i have learnt since using Avast is if there is a problem it is always rectified very quickly. A reason why i have always trusted, and continued to pay for my renewal to IS on a yearly basis. A reason which i suspect the OP chosen too!

Maybe the OP should just relax and figure out his next step with his customers.