How much times can be used a Windows 7 serial key?

Dude, just do a quick Google search instead of posting here...

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AFAIK the serial can be reused multiple times however it can only be used on one computer at any time. If you activate the serial on your computer it will deactivate his copy of Windows.

one time. it costs so much because so many people want it and it's expensive. use linux if you want a free os

So seems maybe not :S, but if I go for a big store for example and "get" there a serial key from one of their PCs, and use it on my PC I won't have problems? since their license have to be multiple, because I heard some Windows 7 liceses are multiple and normally offices, schools, companies, etc, buy them. In my country I never saw schools, universities, using Windows 7, only in some big stores, and not always, mostly they use Windows XP lol.