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In the meantime, can you please have a look at the attached sample 2003 server settings for the TIU Sample Site? Do tell us what is different in your settings.
Try installing Sitefinity on a Windows Server 2003 and if it does not run, we will have to look at the settings of the Widows 2003 PC. If the Sitefinity sample sites work fine, this means that there is some problem with the settings of your site.
We will also try to change the core assembly Telerik.Framework.dll to be able to trace all successful steps during initializing the application. However, this may take us several days to implement.