At least in the US, fitness for a particular purpose requires that you tell the seller what your purpose is, or that your purpose is implied from the circumstances, and generally you don't tell the seller of your software what your purpose will be. In the case of MS and Windows, you would be talking about the implied warranty of Merchantability.

The interesting question is whether MS, in its shrink wrap agreement, can eliminate the implied warranty of Merchantability, especially considering that most people don't read the license agreement before they buy. This is the classic problem with these sorts of laws, and there is not much legal theory to determine whether these agreements are valid. There is conflicting law throughout the US legal system.