I have an open proxy on my computer that is sending out spam and my ISP told me I have to get it fixed within the next week or they are going to permanently shut my service off.  I don't know anything about open proxy's let alone how to stop it.  I had my sons computer connected to my computer wirelessly through a router and yes it was secured.  I have now disconnected the router from my computer as I'm not SURE which computer has this open proxy.  Can ANYONE PLEASE help me to find out what this is and stop it?  I greatly appreciate any information.  I am fairly familiar with computers, but like I said before I don't know anything about this.  Thank you in advance.
Use a good firewall

As the term conveys, a firewall is an EXTRA layer of security installed onto computers, which restricts access to systems from the outside world. Firewalls protect against hackers and malicious intruders. You need to download a free firewall from one of these reliable vendors.

Remember only install ONE firewall

1) Comodo (Uncheck during installation "Install Comodo SafeSurf..", Make Comodo my default search provider" and "Make Comodo Search my homepage" if you choose this one)
2) Online Armor
3) Sunbelt/Kerio
4) Agnitum
5) PC Tools Firewall Plus

If you are using the built-in Windows XP firewall, it is not recommended as it does not block outgoing connections. This means that any malware on your computer is free to "phone home" for more instructions. Simply put, Windows XP contains a mediocre firewall. This firewall is NO replacement for a dedicated software solution. Remember to use only one firewall at the same time.

Understanding and Using Firewalls

What’s Your Firewall Asking You?