Like PhysX said, the pc was not yours, it was your parents. That was definitely an asshole thing for him to do but he can do what he wants with his stuff. As for the girlfriend, get over it. I'm not trying to be mean or anything but you're in high school. High school relationships are not real, you do not love her, she does not love you. Period. Don't argue. They are great practice for later in life when you have matured a little more and know what you want, but right now no relationship is going to be serious or long term. That's just how it is.
I'm sure this has been said plenty in this thread but I'll say it once more. Your dad needs help. Drunk dads are not good dads and are certainly a bad influence on their kids. I don't recommend you get a job so you can move out asap at 18, you need to focus on your school work right now. What needs to happen is you need to tell somebody what's going on. Is your dad going to get in trouble? Most likely if he hits you and drinks often but something needs to be done. Talk to a school counselor. They are free. Once you're out of high school you have to pay for counseling and it costs a lot. Use what you can get now for free. You can also call the police but I probably wouldn't do that right away unless you felt you were in danger.
....I just went through the thread a little more and read that you were 13 so I guess you're not quite in high school yet, but everything I said still applies. Just pretend the word high school says junior high instead! Whatever you decide to do, good luck.