Why can't I open mail in snow leopard?
I have just installed Snow Leopard and cannot open mail.
Check the console log (Applications > Utilities > Console.app) for any relevent errors that might be reported there.
As a possible fix, (re)download the current combo update for 10.6.8, and see if installing that clears the problem.
Given this is a fresh install and failing the combo update, wipe the boot disk, and reinstall it, and re-import your settings? (I'd wonder if there's a problem with the installation kit you're using, or possibly with your disk, too.)