Thunderbird has its own very efficient and customizable antispam. I turn off the antispam feature in my Norton and let T-bird do its own thing. The email scanning itself is very compatible with Thunderbird.
Google is my home page and works excellently with Norton.
1. NIS provides only partial support for Windows Mail. No tool bar integration & No reclassification as well (Spam/Not Spam)
When you open Antispam->Client Integration UI : Email Client Windows Mail option will be checked by default in Vista. Spam Emails will be moved to Junk Folder
So you need not to do anything to add it
2. No support for Thunderbird this time too. But still you can follow the steps given by Yogesh to filter Spam Emails
3.
Too Symantec Rep...suggest adding a feature similar to Windows 7...
here's what's new (which you have) and here's what's not supported any
more. - 70f9