IIS 7 Service Unavailable 503 ERROR

I am encountering a 503 - Service Unavailable error. I checked the event log and found this:

The Module name FastCgiModule path C:\WINDOWS\System32\inetsrv\iisfcgi.dll returned an error from registration. The data is the error.

Note: Every time I try to visit my website, I found that the DefaultAppPool will auto-stop. And also found this in event log:

Application pool 'DefaultAppPool' is being automatically disabled due to a series of failures in the process(es) serving that application pool.

How do I fix 500 Internal server error in IIS?

IIS error The error 500.19 is an internal server error often occurring on a server using Microsoft IIS software. It indicates that the configuration data for the page is invalid. To solve the issue, delete the malformed XML element from the Web. config file or from the ApplicationHost.

This is because your application pool is crashing more than 5 times in 5 minutes [default settings - Rapid Fail]

Instead of disabling Rapid Fail, you should consider taking Crash Dumps and try to find out the root cause. http://blogs.msdn.com/b/rahulso/archive/2006/03/02/what-is-a-crash-technically-in-asp-net-and-what-to-do-if-it-happens.aspx