A person can become addicted to anything. We all have our vices, and some people seem to have more of a weakness towards certain things than others. However, using a computer or using drugs doesn't become bad until it starts negatively affecting your life (i.e. you ignore responsibilities, stop sleeping properly, stop socializing, etc).
I don't think computers are really comparable to illegal substances, however, because they involve a physical addiction as well as mental--and you go through withdrawals. While a person may feel depressed if they can't use their computer (which is a mental addiction), there are no physical symptoms that go along with that. All you have to do is find something else to occupy your mind. I agree with Angela Morgan that it's really the pleasure that you receive from computer use that is addictive, not the actual computer itself.