Designing usable Graphical User Interfaces (GUIs) is a complex process and requires at least two things: knowledge of known GUI design principles and guidelines, and knowledge of structured methods for achieving usability. The purpose of writing this thesis was to investigate whether a GUI standard increases a program's usability or not. In many cases, people who take part in GUI decisions do not consider following a standard and the decisions are often based on what is best for them. Consequently, in most of the cases, programs turn out to be inconsistent and less usable. But following a GUI standard does not necessarily guarantee usability. To carry out the investigation, the GUI of a program was deliberately manipulated. Apart from the original one, two more versions were created; one strictly according to the Microsoft Standard, and one that among other things violated the standard. To be able to compare the three GUI versions, usability testing with four test users was conducted. The result of the entire investigation showed that a GUI standard increased the usability considerably. But the result also showed that the test users found the GUI, that in some aspects violated the standard, more pleasant to use, that is to say, users' satisfaction was higher. This indicates that standards ensure consistency, which provides a sense of stability and makes the GUI familiar and predictable. But GUIs should also be tailored for and validated against the users' requirements, and this is done with structured methods for achieving usability. As a GUI designer, your knowledge of the standards is the absolute minimum requirement for developing GUIs. By knowing and understanding the standards, you will also have the knowledge of how to violate the standards in order to achieve a higher degree of usability.