Hi there, I’ve never worked directly with a Windows server but it does sound like that is the cause of the problem. As far as I know, as from PHP 5.3.0 support for ISAPI was dropped. According to the FastCGI website, it works with PHP 4 and 5. I would say you’re safest with the latest, keep everything up to date.
IF you’ve recently updated then this might help, as other people have reported the same type of issue and needed to reset the database password – https://help.fasthosts.co.uk/app/answers/detail/a_id/2005/kw/Internal%20server%20error.
Internal server errors are usually described in more detail in the server error log. If you have access to your server error log, generate the error again, note the date and time, then immediately check your server error log for anything useful that could point to what’s happening. If you don’t have access to your server error log, ask your hosting provider to look for you and see if they can pinpoint what the cause was.