The primary goal of most OSs (Operating Systems) is the efficient use of computer systems software and hardware resources. Since Windows OSs are most widely used OS for personal computers, they need to satisfy needs of all different kind of computer systems users. In comparison with Windows XP, new versions of the Windows OS; namely Windows Vista and Windows 7, introduce a number of new features and enhancements. Furthermore, performance improvement was imposed as one of the key design goals for both Windows Vista and Windows 7. This paper presents a performance evaluation of three latest versions of the Microsoft OS for personal computers; namely Windows XP, Windows Vista and Windows 7. OS performance measurement is done by means of a set of benchmark applications in the controlled environment. To ensure accurate, reliable and repeatable performance measurement results, we have created a performance measurement process and a performance evaluation model. Special emphasis is placed on evaluation areas with the greatest impact on the performance: CPU cheduling, memory management, graphic subsystem management, hard disk drive management and network performance. To determine the Windows OSs performance in different environments, performance measurement is done in three experiments. Experimental results indicate that Windows Vista and Windows 7 have several performance improvements on the stand-alone high-end computer system, but Windows XP outperforms Windows Vista and Windows 7 on the stand-alone low-end computer system. Furthermore, on network computer system Windows Vista and Windows 7 show network performance improvements mostly for the traffic with medium-sized packets.