the 500 error is a very generic error that literally means "An error occurred". It is a generaic error that will show if the system has been set not to send errors to the screen.
to be able to tell whats wrong, we need to know the real error.
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.
This will enable debugging info to display when an error occurs. This should be turned on while testing and should not be left enabled for a live server.
Also, try using FireFox instead of IE.