On my Dell Studio 17 I had Vista Ultimate x32 pre-loaded.
I used a Vista Ultimate 64 bit CD to wipe it clean and hand-install.
Interestingly enough, even though I checked the key/serial (that was on the sticker on the PC) re-checked and did it all over again, when the system went to auto-activate, it failed.
So, I put in the same serial AGAIN after it failed , and voila - it worked!
This is not the first time I've seen this though. My wife's desktop PC had Vista Home Premium on it, which I had to wipe in favor of WinXP Pro because of some incompatibilities with apps she was using for school. When she was done with those classes, I put Vista back on, and the exact same thing happened.
Anyone else seen this behavior?