boy - i think I'd make separate user accounts for the whole computer.

make your account the administrator - you can make major changes
make his a limited account - he can't make major changes..

then you would both have your own setup and your own copies of programs.
___

Or you could use the Thunderbird mail program for your self only -and he can keep using the Outlook Express program.
I don't remember if you deleted it or just stopped using it.

I really couldn't guess at what he might have changed to lose your email identity.
unless you go thru the settings if you think he was messing around in those- he must have been...