Thunderbird crashes 0xc0000005 exception

We've been fighting this for almost a year now - I've contacted Thunderbird support forums but they just don't know what to do about it:

My wife's (Mozilla) Thunderbird frequently (8-10 times a day) crashes. The error is a code 0xc0000005, and the error signature shows Application Name= Thunderbird, Modname = msvcrt.dll.

We did a clean install of TB 2.0 about a week ago, didn't help a bit. I've run System Mechanic, ccleaner, etc, still no help. We're using AVG Internet Security Suite (firewall, anti-virus, etc) and haven't been notified of any virus problems. I know this is gonna be a time-consuming fix, but we have to either find the problem or I'm gonna have to reformat/reload everything (ugh!) -

Can anyone help me track down the source of the problem? Thanks very much -
steve