Outlook express is very old now and has none of the accepted security measures that would be expected from a mail client these days.
And as far as I know you can't run it natively under windows 7 anyhow.
the closest other mail client to OE that I'm aware of is evolution,
http://www.dipconsultants.com/evolution/
But thunderbird is probably easier and has more support for transferring mail between different clients.