Discuss this firmly with Microsoft. If you have paperwork, or certificate of authenticity, or invoice, or receipt, for the computer or the Windows Install, they will help you by giving you a special install number. Do not expect this to happen fast... but get all your ducks in a row. They will want to know what, where, when, why, and how...
If you have nothing in hard copy or on your computer case, or any other such stuff, you are screwed... They didn't steal it from Microsoft. They stole it from you or from the computer manufacturer, or retailer.
When you call Microsoft, have all your stuff ready to fax, or send my photographic image.
They have an interest in stopping this.
We have worked with Microsoft on several such issues... and with appropriate proof, they have always been helpful... though it took several days.
You will have to pay a small fee to fix it... under $30.
There are rings of these people in Mexico and Thailand that flood the US market. and there are always bad apples in the retailer community... Microsoft is interested in stopping it.
Since they have limited your privileges, you can count on the fact that there is more than 1 copy... perhaps as many as 11 copies out there.