So the other day I broke my event viewer in Vista...

And I couldn't find a way to fix it and when I used my full version Vista media to try to do in inplace repair, the only option it gave me was to do a full, clean install wiping out my data... hmmmm no so good... so I got a copy of Vista Ultimate upgrade media (this is a tablet PC) to see if I could do a inplace repair.
Sure 'nuff, when I had upgrade media, it did what I needed it to do, repairing my existing install and fixing the event viewer.
When it rebooted, I did have a smidge of reactivation to go through, and until I rebooted it had "you are not genuine" on the screen...

And I had to go through the phone call activation as the wireless button mouse wouldn't turn on the internet while the machine was in activation mode... but the moral of this story is?
I think upgrade media is MORE flexible than full media. Remember the trick of installing cleanly with upgrade media? Install it without a product key, then install it again with it?
Truly the only way it appears I was able to repair my event viewer problem was to get upgrade media and use that, not the full version media.
Needless to say I'll be playing around a bit in vmwares to see if "fixing" Vista is always this way.