Directed to you. Avast installs the Add-on, not you.

Humour me. Just do it.

Open IE > Tools > Manage Add-ons

Are there any lines there for “avast! Online Security?” If so, is the status “Enabled” or “Disabled”? If it is “Enabled” change it to “Disabled”. Right click anywhere on the entry and select “Disable” from the drop menu.

Then close and restart your browser and see if your problem persists.

If the status is “Disabled” or the entry is not there, then that’s not the problem, and I’ll butt out.