As the sole authority (network admin) at my previous job, I uninstalled IE on every PC on campus (about 170 of 'em) and installed Firefox. After some initial grumbling from people who were, shall we say, resistant to change, it was universally adored.
For the novice/casual user, Firefox with the Ad-Block Plus add-on is enough to to make them forget all about IE. Imagine not seeing ads in your browser. I'm not just talking about pop-ups, either. I mean all the in-line ads on any given page. If an ad somehow slips through, you can easily right-click it and add the offending ad to the blacklist, and you'll never see it again. With a bit of practice, you can forget the individual ads and block the servers were the ads are coming from. It's cooler than the other side of the pillow.
For the advanced user, the flexibility and power you can achieve with the various other add-ons make Firefox a must-have application.
As was mentioned previously, you can download Firefox for free at http://www.mozilla.com/firefox