[Hampshire] Banishing Windows Mail - Can anyone help?

Hello,
I have just installed Thunderbird on Vista (through a long and
complicated chain of events - well alright I accidentally deleted the
link on my Linux Mint box that helps change the internet settings -
ahem... I am waiting until the new release of Mint - Felicia comes out,
then 'starting afresh').
I am trying to get rid of Windows Mail/Make Thunderbird default, and I'm
sorry to say I have no idea how to do it.
Please help!
Thanks
Becky