Want to download a Windows 7 product key generator? It's actually pretty easy.
I came across this video with step by step instructions, and I started following the precisely (skip the first minute, he's just rambling for no good reason - nothign to do with Windows 7 product key generators)
The thing is that if you need to get hold a Windows unique product key (CD key) or serial number, the legitimate ones are always found on the CD or DVD that Windows came in.
You may have lost that though - so WIndow's has a second option: The Windows Registry.
Obviously, if you never owned a genuine Windows product before, that will be of no use - which is why people resort to generators as the one mentioned above.
However, it's not the only one in the market - and since you're talking about an illegal black market when you go into key generators, it's hit or miss whether they will work, fail, or infect your computer with malware.
I know that might not be what you want to hear - but it is the truth. All the other sites that don't say this just want you to download their scammy programs, but at least I am not afraid to say that some people that write these programs out there and make them available for free probably have an ulterior motive.