Denise,
IPCop split off of Smoothwall. I use IPCop in our organization because they have a stronger open source spirit imho.
There is an add-on called url-filter which works well, as well as adv-proxy. These add ons are avialable for both IPCop and Smoothwall. Both of these add ons legally require that you notify your students and instructors you are using them and that thier internet usage may be monitored. Employees, students have the right to know you are monitoring them (resonable expectation of privacy - which is in the grey on interent).
CopFilter is an add on that uses has an anti-virus and spam software built in as part of the add on and blocks them before they have a chance to come into the network.
A split off from IPCop is Endian Firewall that looks promising. I have delayed using it until it comes out of release canidate mode.
I work in an environment where my time and resources are limited so i chose ipcop to do my dirty work for me because it was easy to set up, easy to keep running without a lot of fuss.
I am currently running a 266mhz machine with 128 mb ram and four d-link network cards. Red, Orange, Green, and Blue. Red - Internet, Orange - Web Server, Green - Internal network, and Blue for the wireless network.
I also use ZoneCd (PublicIP.net another open source freebee) to offer public access which is filtered by Dan's Guardian for wireless access in my customer lounge.
Dan's Guardian is by far one of the best content filtering solutions that I have seen.
Don't have a old PC? Walmart sells em on the cheap (150.00 or so) that should work well enough for any firewall/filtering solution.
A word of advise. Test. Test the firewall setup/connection, the filtering solution, and any add on. I have been in the uncomfortable position of explaining why the internet connection was down for an hour while trying to figure out what add on broke the firewall.
Here are the links and their screenshots.
www.smoothwall.org www.ipcop.org - check thier add ons www.copfilter.org www.efw.it www.dansguardian.org