Avast disables Thunderbird-fix suggested by FAQ works but not permanently

I read the article (http://www.avast.com/faq.php?article=AVKB91#artTitle) on how to fix this and have applied the steps three times. It works for a while and then after a few days or so, the problem comes back as if the fix had never been done. Is there any way to make the fix permanent? I am running a Dell 4600c with Windows XP Pro and Thunderbird v. 17.0.7.4919