Wel, he can hack into your system yes, but... Your Laptop was obviously hidden by either a proxy server or a router (That is why you saw a different IP address when you looked up your own IP). The Proxy server does everything on behalf of your computer and if you are behind a proxy server then he must first get past that server to penetrate your system, the proxy server acts as a firewall. Depending on the proxy server's security your laptop will be quite safe. Thats if you are behind a proxy.

The other alternative is that you are behind a router that also can act as a firewall and is very secure (most of the times). The router will basically require the hacker to give some of pre-authentication before he can hack you, the pre-authentication just verifies that he is allowed to use that network and the network will accept him if he gets authentication. That can really make things more difficult for him.

What you can do now is the following:
Do not use a static IP, use dynamic IP.
If you have a password, remove it. (What can he crack if there is nothing to crack...)
Do not Log on with your Administrator account username, create another Admin account with a different username.
If you prefer to use a password and your normal admin account, make it a strong 10-14 characters mixed-alphanumeric password.
Keep your firewall on and do not allow any exceptions.

If you keep by those guidelines he wont be able to hack you, it is possible to make a system unhackable. You can also download a little program called "Who's watching me" that will tell you once someone is busy cracking your password with a remote cracking tool.

Astalavista!