With the PC market flagging in general at the moment due to tablet sales disrupting what was once a reliable PC upgrade cycle, many Microsoft (MSFT 0.05%) investors were hoping that the company's new operating system, Windows 8, would rekindle consumer interest and drive a lot of PC sales. However, consumer market research group NPD recently released a study saying that Microsoft-based PC sales were down by more than 20% over the last month. In this video, Motley Fool analyst Andrew Tonner talks about why this could be a real red flag for Microsoft investors, and may affect the investing thesis in the company. He also highlights some potential margins for error in the study itself.