Outlook Express stopped working for me a year or so ago out of the blue on WinXP Pro, so I switched to Thunderbird and never looked back. I was already using Firefox, so it seemed a natural thing to do. I see that others have answered some of your questions, but keep asking if you have more. Someone can certainly help!

As for calendars, I use Google calendar and like it a lot. It's not integrated with Thunderbird, but since I use gmail and iGoogle, those all work together nicely.