Here's my take on things:
Firstly, At 15, you have a right to be a moody bastard, but from what your saying its probably more than just that...
Dont bother with a doctor unless its a real problem... They tend to want to put you on meds, and that just makes things worse a lot of the time.
A lot of anger can be habbit. Sometimes its a lot more satisfying to just relax about stuff. When you get angry it puts a lot of strain on you, so try and learn to let things go. In the end, how your feeling inside is most important.
As for your computer, I really cant stand slow computers either! It all depending on your level of technical skill, theres plenty of things you can do to make things better. Prevention is better than the solution though, when your surfing online, make sure you dont accept ANYTHING your not sure of, and if your in doubt, just close the window. (With the cross, or hold alt and press f4) - Trust me, there are about a million guys online just ITCHING to show off their computer ego and help you with your problem, so ask, the help is there!
Finally, Lose some weight fatty
(lol, just kidding)