I was in the same situation some time ago. I was changing my system from XP to Vista, and I used a new hard drive for that purpose. All other hardware stayed the same in the PC. Upon installing of
Microsoft Streets and Trips 2008 the activation didn't work (as I had previously used up both of my allowed activations).
If I recall correctly when the activation failed it prompted me to telephone Microsoft Technical support (a phone number was included). I phoned them and they helped me activate it over the phone. It was quick and painless

.
I was told that if there was no hardware change the activation would succeed.