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.
				
			 
			
		 
			
				
			
			
				Emily
2011 Jamis Dakar XC "Toto" - Selle Italia Ldy Gel Flow
2007 Trek Pilot 5.0 WSD "Gloria" - Selle Italia Diva Gel Flow
2004 Bike Friday Petite Pocket Crusoe - Selle Italia Diva Gel Flow