internal error 500 is a generic error message.
Are you testing with IE? IE has a setting to show unhelpful error messages. If you are testing with IE, go to Tools -> Internet Options on the advanced tag, uncheck the Show Friendly HTTP Error Messages option. You may also want to try testing with another browser like fire fox.
If you are testing locally, it is possible your server is set to suppress error messages.
To enable the more advanced error messaging in IIS 7 for Vista you will need to do the following:
1) Go to Control Panel > Administrative Tools
2) Open the IIS control panel and open the ASP option
3) At the top of the ASP control area select Display: Friendly Names
4) Now go down to the Compilation area and expand Debugging Properties
5) In debugging properties make sure to set Enable Client -side Debugging, Enable Server -side debugging, and Send Errors to Browser to TRUE. Make sure to click the Apply on the right side of the page.
6) After setting the debugging you must go back to the IIS control panel and using the controls on the right side restart the server.