An appeal to the user to use a safe and standards-compliant browser instead of IE.
Get a better browser
I don't want to harp on and on about web standards here. Suffice it to say that standards in most any field make development of the medium easier and faster. I know most people don't care, and that's fine with me. This web site should be accessible in any browser. The layout and design will look their best in browsers that support web standards, such as XHTML and CSS, but it should degrade gracefully in browsers that don't.
Since the main problem browser in my experience is Internet Explorer on Windows, I'd like my readers to know a couple of things about that browser. First, it's old technology. The last time that Microsoft has made a major update to IE was years ago. Only recently have they added the ability to block pop-ups with Windows XP Service Pack 2. Meanwhile, the Mozilla project has been hard at work with Firefox, which has features like Tabbed Browsing, a Download Manager, better blocking of pop-ups and other JavaScript annoyances, Secure Password Management, an integrated and extensible interface for various search engines and a whole host of optional extensions that increase the feature set further. Opera has also developed a strong feature set, though I don't use it often, so I'm not as familiar.
Second, and perhaps more important, is the issue of security. Because of ActiveX and other holes in Internet Explorer, an uninformed user is at great risk of being infected with some kind of malware while using the browser. Of course, any application is susceptible to problems, but the track record of IE is simply inexcusable.
So my recommendation is for Firefox for just about any platform, but there are plenty of others out there. There are other browsers based on the Mozilla Layout Engine (Gecko) such as Mozilla itself, Netscape, Galeon and Kmeleon. Then there are the KHTML-based browsers such as Konqueror and Safari. So whatever your platform, there's a good browser available and no excuse for using the dated security and standards nightmare that is Internet Explorer.