I understand the addiction – and its a tough one! I would say harder than the “big” addictions because those are much more obvious.
If you are really having trouble controlling yourself, and dont need the computer (for work or anything) – I would give it up.
First though, set realistic time limits for yourself. Lets say 30 minutes twice a day? Then get a timer out, and set it for 30 minutes. When its up, its up. If you really want to do this but dont have the strength and have windows Vista, you can program it to do it for you.