Why are all of my ports closed?

I am running Windows XP Pro and I cannot find an open port. I disabled my firewalls and disconnected my router, so I have my PC directly connected to my modem. I even installed a fresh version of XP home, and still had the same problem. I finally called my ISP and they are not blocking me. I have searched every help forum and site I could find and apparently no one has ever had this problem.
Anyone have any ideas?