Actually, as posted by jimbo45 some time ago..............
As long as you have a Vista machine, you can copy the contents of the C:\program Files\Windows Mail directory from the Vista machine and replace the files in the C:\Program Files\Windows Mail directory in Windows 7.
You'll need a program called ]Unlocker1.87 to delete the files on the Windows 7 machine.
Otherwise I'd recommend the open source thunderbird 2.0 e-mail client
I think its sad windows 7 lacks a built in mail system, though it will encourage people to move to web mail more, this will make moving to new systems after windows 7 less of a nightmare.