That was the first thing I did. Sorry, being a computer shop owner, I should have posted some of my troubleshooting in the original thread. Long day.
I've uninstalled/reinstalled both Thunderbird and Avast multiple times. Once with Avast first, once with Thunderbird first (thank goodness I know where my profile folder is so I didn't lose any mail/contacts!) and it still did the same thing.
I am an Avast reseller, and since I install Avast on new computers I build, having the knowledge beforehand (since it's happening to my computer) will be handy in the field.
Also, I'm on Win 7 Ultimate and Avast/Thunderbird are both running as an Administrator.