There are lots of people using avast with IE8 without problem, but if they are using a 64bit version I don't know.
However, avast doesn't much care what browser you are using as long as it conforms to the HTTP protocols standards (which I assume IE8, 64bit or otherwise is) then it should work. That is speculation on my part as a) I don't have a 64bit OS, nor do I use IE8.
Hopefully someone with personal experience of IE8 64bit will be able to comment.