How can I save my e-mail content (after downloading the e-mail to my computer using mozilla thunderbird), so it won’t be loss if the full windows system needs to be reinstalled.??

I use a different email program, but if mail is saved in the same directory, you can just back up that directory to a cd or another computer. Just make sure that's where the email is stored, you might find that in the thunderbird options/settings.

update: I did a little research on this and found a free thunderbird backup program, it looks like it is used by quite a few mozilla users, I found a lot of references to it. It's called MozBackup. The backup program is at:

http://mozbackup.jasnapaka.com...