The MD5 HASH keys are available on the web. I suggest you check the MD5 hash of the ISO you download with a tool like
winmd5free. If the key matches then your ISO is a non-tampered original.
Win 7 SP1 x64 English Media Editions are 3,243,070KB
X17-58997.iso Home Premium MD5 hash: da319b5826162829c436306bebea7f0f
X17-59186.iso Professional MD5 hash: ed15956fe33c13642a6d2cb2c7aa9749
X17-59465.iso Ultimate MD5 hash: c9f7ecb768acb82daacf5030e14b271e
It is never a good idea to just buy a key unless it is directly from Microsoft.