Windows Live Mail is the new free e-mail client from Microsoft, and can import mail from your Outlook Express mail store directory.
Thunderbird 2 can also import your mails from those files.
I haven't tried Windows Live Mail myself, I moved to Thunderbird back in 2006 when Vista came out.
Either way, make sure you back up the mail store directory from OE before you upgrade the OS. Put it somewhere external, like a CD/DVD or USB drive.
My OE store was too big for Thunderbird to import (many gigabytes and hundreds of thousands of mails). TB churned for hours and I gave up on the import. I still have the old mails backed up somewhere, but I've never actually needed to look at one.