Microsoft Knowledge Base Article 311766 contains the following summary:
The following error message indicates that a problem has occurred on the Web server that hosts the Web site at the time the error is returned:
HTTP 500 - Internal Server Error
This article describes how to troubleshoot these errors. This article is intended for Web site administrators. These errors have a number of causes and can effect many different system configurations. The procedures that are described in this article must be performed by a member of the Administrators group on the server.
End users that experience these errors should notify the Web site administrator of the problem.