I also suggest you follow the advice above.
Go to Microsoft windows updater and ask that they scan for relevant updates. If they tell you that your copy is not valid, go yell [or at least buzz] at the people who sold you the machine and insist that you get what you paid for. If you still have it, bring your receipt with you.
If they refuse, call a cop and notify Microsoft that these people are selling counterfeits of their products.