Accessible Operating Systems
image of onscreen keyboard
Microsoft's Accessibility options in the various Windows operating systems were created for users with visual, mobility, or hearing disabilities. The options provide minimal accessibility, so depending on the severity of your disability, you may need additional software and/or hardware. Following are examples of accessibility options:
Appearance—Users may change colors and contrast on their screens to be able to see it better. You may also reverse the appearance of text on a background from black letters on a white background to white or yellow letters on a black background. Many users with low-vision prefer the latter color-scheme.
Magnification—The magnification option magnifies the top of the page within any program; users need to scroll to magnify each line. Users with low-vision will also need magnification software, such as Zoomtext or MAGic.  They provide total magnification in all programs, as well as screen reading capability. Once installed, magnification software programs can be configured to automatically open each time you start up your computer.
Hearing loss options-- People who are deaf or hard-of-hearing can configure Windows to use visual cues in place of sounds, or increase the volume level of program and system sounds.
Sticky Keys—designed for people who have difficulty holding down two or more keys simultaneously. When a shortcut requires a key combination, such as CTRL+P, StickyKeys will enable you to press a modifier key (CTRL, ALT, or SHIFT) or the Windows logo key, and have it remain active until another key is pressed.
More mobility options-- People who have motion-related disabilities can use Windows to configure their computer system to provide a more comfortable computing environment and help minimize the impact of mobility impairments on their work. The Language category in Control Panel, provides keyboard layouts for users of alternative keyboards. Here's a link for Windows 8 telling how the keyboard can be changed to Dvorak, a keyboard often used for single handed typists. This is especially useful for those who type with one hand. The Mouse section in the Control Panel can be used to configure the mouse for right-handed or left-handed users, adjust double-click speed, pointer speed, and acceleration, and send the mouse pointer to go directly to default buttons in dialog boxes.
Onscreen keyboard -- Some people may find the use of the onscreen keyboard helpful.  This is a "virtual" keyboard that appears on the screen, much like you see in tablets.  Options lets you select by hovering over the keys or clicking.  Text predictions can also be used. These directions tell how to turn on the onscreen keyboard in Windows 8. 
Tutorials for Microsoft accessibility can be found here.