First, make sure Windows is completely up to date. Download and install any updates you may be missing. Next, try repairing IE from the add/remove software control panel. If that doesn't work, try completely uninstalling you AVir and reinstalling it along with all it's updates.
Note: you may need to download a separate utility from your antivirus publisher's website to get a really clean uninstall. (BTW - did you check support on their website to see if this is a documented problem? Usually they will have a fix if it's something caused by their product which they know about.)