If full retail version of Vista, you can install it & the product key code onto another system - as long as the one it was installed on is no longer "live".
I would not recommend using an OEM version of Vista on any system other than the one it came pre-installed on. I have seen problems surface weeks/ months later. I don't recall if the OEM was the same, though.
The retail version installs on different systems I have done myself - used the same product key code on a different system after the other system was inactive. Also, a retail version of Vista contains ALL versions. It is the key code that dictates whether Home Premium, Ultimate, etc... will be activated and then validated. If you install Ultimate on top of Home Premium w/ the H-P key, it considers it to be an upgrade that you will have to pay for at a later time when it asks for the newer versions' key code.
Have you had different experiences?