Hello Darryl Clark,
I am afraid that the ComboBox control that you use is form the RadControls for ASP.NET suite - this is an old suite that is not currently supported.
Please find more details about the RadControls for ASP.NET Product Support Lifecycle here.
Additionally the version of Windows Server (2003 or 2008) is not related to the way RadComboBox is displayed on your page.
You can check the Internet Explorer version installed on the server in order to detect under which browser the issue appears. Then you can try to force the page to render in another IE version mode.
For example you can emulate IE7 with this meta tag:
Or you can try to force IE8 with this meta tag:
Please note that you have to add only one from the above meta tags on your page.
However, the best that you can do is simply to upgrade to RadControls for ASP.NET AJAX
the Telerik team
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