RadEditor Lite for MOSS is based on RadEditor Classic
which does not support Internet Explorer 8. The development of RadEditor Classic was discontinued in favor of the new and much enhanced RadEditor for ASP.NET AJAX
more than 1.5 years ago. The Ajax based editor is the core of the paid version of RadEditor for MOSS.
The recommended solution is to upgrade to the full featured and paid RadEditor for MOSS (based on RadEditor for ASP.NET AJAX) which offers full support for IE8, multiple bug fixes and many new features. The new editor loads multiple times faster than its predecessor as well. The upgrade instructions are available in this help article.
If you are unable to upgrade to the new editor at the moment you can try the following code that should be put under the RadEditor Placeholder declaration:
(window.RadControlsNamespace.Browser.IsIE && document.compatMode)
You can use the SharePoint Designer to put the code in the page with RadEditor.
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