the HTTP 500 is a generic error that just translates to "An error has occurred". We will need to know the specific error that has occured to trouble shoot the problem.
Are you using IE? If so try using FireFox.
In internet explorer there is a setting to "Display Friendly HTTP Error Messages". This setting is turned on on your computer so you are not getting the full error. To be able to debug the problem Friendly Error messages will need to be turned off so that IE will display the actual error.
In IE go to Tool -> Internet Options. Click the Advanced tab un check the option to "Display Friendly HTTP Error Messages"
Are you testing locally, in IIS 7? If so:
To enable the more advanced error messaging in IIS 7 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.
once we know the actual error, we can try to figure the cause.