i too used to be addicted badly to my computer. right now, ive been spending less time, albeit more than your average person.
I find boredom to be an underlying factor. Lets face it, the reason why i was spending hours in front of my comp doing useless useless stuff that i dont even remember the next day, is because i have nothing better to do....quite sad really hehe.
If you want to stop being so addicted to your computer, find a hobby like join a sports club, go out with your mates, etc.... It has worked wonders for me.
It's just a matter of finding a more meaningful and useful way to occupy yourself.
Im not saying boredom is the cause of all computer (or any other) addictions, but it certainly was in my case and im sure in many others as well.