I was sold a computer with no product key for the ...

I was sold a computer with no product key for the operating system. I am told there are many other computers like this one in for servicing. The computer stinks, files are corrupt, the guy I was speaking with said he can't help and hung up on me. I can return the computer is all. How about all the time I sunk into it. I have to return the computer because it came with no operating system product key. This is supposed to be whose fault? This is a horrible store, like a nightmare.