If your torrent copy is from a good source (ie unmodified), there is no difference between it and a retail copy bought at a store. A valid key would work.
Why risk it ? The torrent version is probably one of the so-called "pre-activated" versions which simply means they install a loader for you or you can use a loader. Buy your copy and keep the key until you install from it. But to answer your question - the key should work several times on the same hardware and then will continue to work but only by phone activation. So no real concerns provided it isn't tried on different machines - which can be alright for Retail versions but will probably require "phoning home" to beg for forgiveness.
Are you serious? Have you even read the title ? If not read: windows-7-rtm-build-7600-16385-original-untouched-msdntechnet