I'll start by saying I liked Outlook Express, got on with it well, preferred it to Thunderbird. Vista's Windows Mail I fell out with instantly when it made a mess of importing my WAB, creating multiple copies of every contact. Also its address autocomplete doesn't work as an autocomplete should. Other moaners suggested Live Mail, seems a bit better thought out but MS have still not improved on the original. I may have to have another look at Thunderbird...
I use Outlook 07 personally but I've set up a few accounts Live Mail for other people and have had 1 or 2 problems with multiple copies of emails downloading and sorting in the wrong folders but wasn't sure if that was the app or the mail provider.
I have to say that Thunderbird does everything i want it to do and more... I use Gmail as my account and that i have set up as archiving everything i pull fromk the server. Thunderbird handles the filtering and sorting of email like a charm..
OSX Mail = true win. Thunderbird is nice (adds are very useful), but Mail feels more streamlined, responsive and less bloated. For the PC, I would go for Thunderbird. Ninja edit: i didn't notice it was for PC, ops sorry.
That post gives me a warm fuzzy feeling to think that people are starting to think outside the MS / Win paradigm. Suddenly we're NOT all using the same thing, and I think it's wonderful.