Windows live mail is built in to 7, and works okay, the ribbon bars are a 
different layout.  Office 2003 and above will work on windows 7.  I don't 
know what screen reader you will use, but the versions back a couple of 
years on most will work under windows 7.
Think of ribbon bars, as two parallel menus, each choice has options that 
you tab through, right and left arrows are used to navigate through the main 
choices.  Up and down arrows will switch between the two ribbon bars. 
That's my image of what is going on anyway.  You can import your settings 
and addresses from outlook express in to windows live mail.  They can then 
be exported to files, and imported in to windows live on the 7 machine, if 
that's the way you choose to go.