Vista uses different encryption or activation thingys. I tried getting one retail onto two computers, and it didnt work out at all.
They keep a file of how many activations you do. You can get two activation within three months of purchase, then the records are reset. It has to be first unistalled, then installed on a diff computer though. If you max out your activations, then you can just call them and tell them that you fried your mobo.
Or you can just use a cvack.