iframes are only unsupported by Netscape 4. Netscape 6+ renders them fine. Here's a solution that will probably work for you:
Originally Posted by okephoto
This way, the iframe will show in browsers that support it. In old browsers, users will see a link that opens in a new window. And for text-only browsers like Lynx, the link will open in the same window. There, everybody's happy!
<iframe src="http://www.example.com/affiliate/" width="200" height="60">
onclick="window.open(this.href, 'newWin', 'width=200, height=60');
Link to affiliate