Windows, in any incarnation, isn't free. It comes with nearly all pre-assembled computers. The computer manufactuers (called OEMs, or Original Equipment Manufacturers) don't get Windows for free. So neither do you. The cost is secretly added into the cost of the computer.
Sometimes a store will sell products with the tax included in the price. You're still paying sales tax.
At some restaurants, you might not leave a tip but you may still be paying for one. Some restaurants include the the equivalent of a tip in the cost of the food.
If the receipt for your computer says the computer cost a thousand dollars, it's not totally true. Some part of that thousand is actually the cost of Windows.