A very competent machine at a good price. Windows 7 Professional 64 Bit OS was preinstalled, and a disc containing Windows 8.1 was included in the box. However, I went directly to Windows 10 taking advantage of Microsoft's free upgrade offer. Apparently because this model includes all up-to-date hardware, it seems so far to be completely compatible with Windows 10. It is successfully networked with several other machines running a variety of Windows releases including XP SP3, 8.1, and 10. All are working together nicely. Replaced the 2 TB HD with a Samsung 850 SSD, and relegated the included HD to "Disk D" status for convenient additional data storage and backups. With the SSD installed, it starts up and is fully initialized within less than two minutes, which is very good as compared to other Windows 10 machines. Were there a need, I would buy this box again.