You can't operate your computer effectively when your hands, arms, shoulders, or neck are fighting with you. Painful muscles and nerves are your body's way of telling you that you are doing something wrong.
To prevent repetitive strain injury, take frequent breaks, and pay attention to your posture and the way you hold your hands over the keyboard. Keep your wrists level, and your hands in a neutral position. When you use the keyboard, move your entire hand and arms to distant keys, instead of just reaching with your fingers. Choose a chair that supports your lower back and arms, at a height that allows you to move your knees under the keyboard. Place the monitor screen at eye level, and adjust the contrast and brightness for the most comfortable levels relative to the ambient light in the room.