A Desktop's form factor describes the physical size and shape of the system. In general, towers and larger form factors offer more space for add-ons and cost less than smaller counterparts.
The operating system, or OS, performs basic tasks like running software applications, files on your hard drive, and directing peripherals like a DVD drive and directly affects what you will see and how you will interact with your computer. Operating systems include Windows98, WindowsXP and many others.
Operating System
Microsoft Windows Vista Home