2
Assuming you're not behind a firewall on your PC, you could use
GRC's Shields UP! to do a comprehensive port scan on your system. If you do have a firewall, you could open the ports you are interested in and run Shields Up! again to see if your ISP is blocking them.
Common ports that you may or may not want open depending on your level of knowlege and what you're trying to accomplish.
Port | Ussage
=============
80 | HTTP *must be open to host from your house w/o a proxy*
25 | SMTP
110 | POP3
21 | FTP
22 | SSH
