If you have your original Windows disc, try this:
Start>Run>type "sfc /scannow" without quotes and hit enter. It will ask for the original Windows CD, and will replace all your system files back to original. Also know that some viruses in the past have used this process (or a process that looks very simular) so I would recommend also running a couple of spyware apps. Your firewall is probably not the only thing messed up
Other programs may be as well....