Each of these is a symptom of eye strain, and the longer you spend on the computer, the more likely you are to suffer. Fortunately you can follow ten simple steps to prevent short term eye discomfort from becoming long term eye damage. Some unions say take ten minutes break every hour, but I'd recommend that every 15 minutes, you get up, stretch, and look out the window. This gives your body and visual posture a break and helps you concentrate better. Glancing away from the screen into the distance every few minutes will also make work at a VDU more comfortable.
Glare is a major cause of eye-strain for computer users.
To detect unwanted glare, sit at the screen with the VDU off. If you can see lights reflected off the screen, then you've got to remove them. Draw curtains on windows near your computer to limit direct sunlight hitting the screen. Use a side light at night, and lower the general room lighting. Check that the clothing you wear when you work isn't reflected in the screen.