Helping a friend to configure his new Win2k8 Server with IIS7 this weekend we ran into an issue where IIS kept returning 503 errors.

Examining the Application Event log I saw that  IISW3SVC-WP was quitting due to application errors. "The error is the data".  How helpful.

After a considerable amount of digging I discovered that in the Microsoft.Net/Framework/ folder, there was a beta version of v2 of the framework.  The only version that *should* be there is v2.0.50727. 

I deleted the beta version of the framework and restarted the IIS worker processes that were causing problems and everything immediately burst to life!

I've no clue what installed the beta version of the framework, but it is a definite lesson to make sure that when distributing a framework to be 100% certain that you are always including the latest "release" version... oh yeah, and perhaps doing a check for an existing version of said framework before installing.

At any rate, hope this helps somebody.