Download the programs Doug linked to, they are the 'standard' package, and run them. Take note however to not have more than one anti-virus program installed. Update everything, windows, antivirus, anti-spyware etc.
If your system is badly infected, you are facing a reinstall.. If you dont update your system after it is reinstalled, I give it 15 minutes online before it is infected with something again.

A third party firewall installed on your computer is not something I would have used. The buildt-in firewall in Windows XP is "good enough". If you want something more, buy a dedicated appliance unit from e.g. Linksys and get wireless at the same time.
Consider what browser you are using tough, it is a fact that certain browsers are more vulnerable than others when browsing. I am using Firefox (http://www.mozilla.com/en-US/firefox/), and are pretty happy with it.

Your ISP have probably shut down your service in 'self-defense'. Your computer is probably used to send spam e-mails and as a platform for attacking other computers. You need to convince them that your system is OK again before they will open it. A re-installed system should be good enough. If you are going to get a firewall, install it before going on-line again.

I have two different firewalls in front of my home-network, and run antivirus/spyware/malware/adaware scans on all computers every day. Still the family manage to drag nasty stuff on to our computers from time to time, without really trying.. The internet have become very darwinistic.