Once you have installed and activated the game, the activation information is stored on your computer and you will not need to enter it again (even if you reinstall the game), as long as you download and install from the same site where you purchased it.
I'm glad it's working again. One thing you should be aware of about Windows Vista is that it has extra security features to prevent changes to installed programs. I have described one facet of those features in the "Special note for Windows Vista users" at the bottom of
this post. That security feature may have played a role in the difficulties you were experiencing last night.