automatic proxy settings windows xp

Our schools has a private company who arranges everything with computers
they supply the computers with special settings
For surfing on internet we use this proxy url
http://proxy.gial.be:8080/proxy.pac
If we install computers in the school on our own
we always have to give the username
vbx-schools\iaf user
In mozilla I have found a addon for autoconfiguration so we dont get a prompt for username and password(blank)
Is there a possibilty that I can input
http://proxy.gial.be:8080/proxy.pac in the register together with the username
so automatic updating can be done so every program can connect with internet for updates