You could try downloading Firefox on another computer, burn that to CD, then install that on your machine. Firefox doesn't use IE's settings, so if it's just those that's screwed up, Firefox will work fine.

If Firefox doesn't work either, then it's something deeper in the OS settings that's broken - you might want to try reinstalling your connection from the CD your ISP gave you, or even reinstall Windows to take it back to a clean state within one hour