I dug out my old copy of Vista Ultimate to install in a vm under parallels.
After having the virtual disk become corrupted and attempting to reinstall Vista I got the "this license is already registered" Message. Probably because each time a Parallels VM disk image is created, it gets a unique id which windows activation plays off.
After being passed from tech support to customer service to I forget who and finally back to Tech Support, I get told that I am eligible to be helped out, but that I would have to be transferred one more time, and there would be a 5-8 min wait. 35 mins later (9pm - closing time) I hung up.
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Glenn