One of the Thought Bubble post responses sent me on a bit of a crusade to find out more about the licensing for Windows Vista. In particular there *was* as clause that effectively meant that you were only allowed to re-install the retail package once if you changed your hardware. For a lot of people this wouldn't have been a major problem because the OS release cycles are short enough that you might only have two different computers in that time span - but the licensing terms seemed a bit hard for hobbyists who regularly change their hardware.

Microsoft was listening it seems and have revisted their licensing to something much simpler:

• You may uninstall the software an install it on another device for your use. You may not do so to share this license between devices.

Good news!

P.S. Thanks to Frank Arrigo for sending me this info.