A reader wrote in to say that her Microsoft Windows 7 licence key is being blocked, but she's convinced it is legitimate. Our Helproom editor explained how to check that it is a legal install, and how to get Microsoft to unblock it.

QUESTION My PC was lagging, so I did a quick format and reinstalled Windows 7. It would have been fine, had I not accidentally installed the operating system on to my backup rather than main hard drive. I tried once again to install Windows 7, but Microsoft has blocked the licence as two copies now exist. It has refused to unblock the copy, and said I must buy a new licence. I bought Windows 7 legally via Amazon Marketplace, and the fact that it was successfully installed and activated on my first attempt is proof that it isn't pirated. Is there really nothing that can be done? Clare Turner

Due to unscrupulous individuals selling unauthorised product keys, particularly on sites such as eBay, Microsoft has had to block many such keys from activation.

It sounds as though your key was blocked in error, but there is only your word for it that you didn't in fact purchase an illegal licence. In fact, many people unwittingly buy dodgy licences and are none the wiser until the software is blocked. Only Microsoft can determine the cause of the block and rectify the situation.

At answers.microsoft.com, we learned of several customers who had been victims of such a scam, many of whom had bought their Windows 7 licences on Amazon.

Although your copy of Windows activated when it was first installed, the key often isn't blocked until some time after. The operating system will function as it should until the time comes that it needs to be reactivated – during a reinstallation, for example.

A licence shouldn't be blocked simply because the operating system has been installed twice, however. It's more often the case that the key is blocked because it was issued for developer use and then illegally sold on to consumers.

Microsoft has the tools to tell the difference between developer and consumer keys. The best way to resolve this issue is to visit Microsoft's forum. Check out this discussion thread to learn what steps you need to take next.

Microsoft provides a tool that, when run on your PC, produces an output file that you can post on the forum for it to examine. This utility won't reveal your product key, but Microsoft will have enough information to learn why the key has been blocked and hopefully provide a solution.

If you have been sold a developer key, the only legal way to activate your copy of Windows is to purchase a 'Get Genuine Online Kit'. As you purchased the software online, we'd also contact the seller to demand a refund.

For more information on how to spot an illegal copy of Windows, check your physical Windows installation discs and packaging against Microsoft's checklist at microsoft.com/howtotell.

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