There are several free firewalls available, see
=http://forums.techguy.org/t110854.html and scroll down to the middle.
I personally like Zone Alarm and have had no problems with it (although Zone Labs states that their free version does not support networks, so you may have trouble with other computers accessing the files on the computer with the free Zone Alarm).
I've also tried Sygate, which does permit network computers to access files on the computer, but does take some configuring, which can be found on the web (I believe a search of Black Viper on Google may give you a howto on configuration... I'm not sure, so someone correct me if I'm wrong.)
I hope this helps!
