The dissemination of personal computers has posed the designers the problem of developing more efficient interfaces that attend to the necessities of an ever growing diversity of users. At the same time, graphical user interfaces are complex systems of visual elements which meaning or function are not evident. Many approaches to these problems have been suggested, but little has been done towards the comprehension of graphical interfaces capacity to produce meaning. The goal of this work is to understand how Windows XP human interface produces meaning and conveys it to users. To do so, the signs of Windows XP interface where studied under the point of view of Peirce´s semiotics, in their relation to their inner properties, their objects and their interpretants, to the sort reasoning they arise, and to their power to create knowledge. Icons, menus and interface controls where selected as objects of analysis for their diversity and for the paper they play in the guidance of user interaction. This analysis was preceded by a detailed classification of Windows visual elements based on Microsoft´s publications concerning its interface design. A brief description of their characteristics and function was added to their classification. After the semiotic analysis an interaction test with Windows interface was accomplished with the help of some users, which were, afterwards, interviewed. This test consisted in the realization of nine tasks involving the interaction with Windows, which purpose was verify the hypothesis that the signs of the graphical interfaces analogous to that of Windows, for their strong symbolic character, rely much more on their interpretant to complete their semeiosis, and hence, they do not carry out effectively the task of producing and conveying enough information to users.