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Can you describe your environemnt? What OS are you using? What webserver are you using? what version of PHP are you using, and how is PHP configured (CGI, ISAPI, FAST_CGI) and is it using any optimizers (ZEND, Turck MMCache, etc.)? What version of MySQL are you using? If you are using a shared hosting service, you may not have access to all this information.
Also try enabling each debug mode from the admin menu in turn and then try loading the main page. Are there any errors.