There are rare occasions where some people do get systems that have illegal copies on them and its not their fault, however that being said, you would have to contact microsoft and give them that product id to get a cd key. I would however just recommend you buy a new copy via newegg or some other online retailer.
If you're looking for a free copy of windows or a pirated version don't bring your questions here, as tomshardware does not condone piracy and will not help you.
Did you buy the PC from a reputable system builder?
I would go back and tell them that the product key is fake and that they should fix it.
If they don't then I would contact Microsoft to report them and I would also contact your credit card company (assuming you used a credit card) to find out what must be done to reverse the charge to your credit card. Of course, you must return the PC.