If you get an error 500 like this:
The details of the error are:
Http Error 500.0 Internal Server Error
Notification: Executive Request Handler
Logon Method: Anonymous
Logon User: Anonymous
URL: [my site]
Physical path: [path to index.php]
And you're sure you have the correct permissions set then check that the database login has permission to access the EE database.
We got this error when the db user existed and EE had the correct password but we had forgotten to assign any permissions to the user in MySQL.