hmm - I'm thinking you must have some files/parts of Thunderbird left on your system {maybe in the registry}and it is overriding OE opening of the file.

#2 on that other post of mine says if IE got messed up it can affect OE - since we don't know what Alan might have done - that might be an option to try.
Even if you don't use IE - if it still shows somewhere on the computer he might have deleted something or changed something...

I use Firefox mainly too but still have IE available for websites that require it.