Computer trouble. Well my computer doesn't have any more memory to do anything. It freezes up,

Question

Well my computer doesn't have any more memory to do anything. It freezes up, cuts off, and sometimes the screen shakes. People tell me that my computer has a virus or viruses. I don't have any money to get it fixed so what should I do. I tried going to the system restore on my desktop but that said that it couldn't help me so I just gave up. I please need your help