Heres a hyperthetical scenario that some of you are maybe asking. A customer was having some problems - he installed Vista on a machine with a MAK key good for 20 activations. This is a key which supports multiple activations for a business environment. He activated the software as part of the install. He realised after using it that the machine was never going to be able to run Vista properly and reverted back to XP.
What happens to the activation? Does that mean he "lost" a license?
The answer is no - activations are separate to the license. All this customer needs to do in this scenario is to called the Product Activation centre in your respective country and you can get an additional activation credited back to your key.