It looks like the IP address is a Comcast customer. You are probably going to get zero out of them without a subpoena. And without proof that an actual crime happened, that's not gonna happen either.
Your IP address does indeed change. When you boot your computer, your computer requests an IP address from the network, which is provided by your router or the provider, depending on how you are set up. Your IP address "lease" has a time limit, which varies depending on implementation. Most service providers are going to have a short lease, but your IP address stays the same as long as you are online and don't lose connectivity.
Finally, as far as your hijackthis logs, etc, I doubt anything useful will be in there. But don't let me stop you from posting. After all, I thought I was wrong once, but I was mistaken.