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Initially, not knowing whether I was being offered a gift horse or a pig in a poke, I agreed to have Windows 10 on a backup PC, but ignored the UK state broadcasting monopoly's backing of the new Microsoft project and cancelled the reservation after reading a little more about it. A Trojan horse might be a more apt metaphor.
Microsoft say that I would have a month to revert to the Windows 7 that I paid for. Some hopefuls think that WindowsImageBackup would be a lifeline for the existing OS, but as Microsoft has a record of the product key, presumably any machine online could be in jeopardy.