7.5.3 from a known-good installer CD? It sounds most like an Extension gone awry to me. Some suggestions:

Boot with Shift key down and see if it will complete booting with Extensions off.

Is there a networking card installed? If so, pull it and then try again.

You might also try downloading the free 7.5 from Apple again and see if you get to your goal.

Fuller troubleshooting procedure here.