Windows 7: This copy of Windows is not genuine.

Having discovered that my copy of Windows 7 (Ultimate) is not genuine, I am in the process of purchasing the software from Amazon.

My question is a simple one:

When I have the new software do I need to reinstall Windows onto a clean disc, or is it sufficient to enter the key from the new package onto the already installed but non-genuine version?

Generally you can do that and it will work. Depends on where you got the pirated source from as I have seen cases where these copies are obtained from legitimate MSDN (or some other Microsoft) media and there's nothing inherently wrong with them until someone comes along and either gives you a bad product key or cracks it. If you simply were given a bad product key, putting in the new product key should be sufficient; if however the Windows installation came from dubious sources or was modified in odd ways, I would suggest you reinstall as you never know what bad software could have been included in it.

If it is truly not genuine, you do not known what it contains. There may be malware, virus and more.
Do a clean install.

If the key and the DVD came from the same place, clean install.

+1

Clean install, FTW!