If this error occurs, check for a file named "Program" in your C:\ directory and delete it.

For more information refer to http://support.microsoft.com/kb/812486

Gustavo Guerra
I had the same problem... somehow IIS registered a bunch of custom handlers that were taking over control of the applications.

Look for AboCustomMapper-XXX in Internet Information Services Manager -> Default Web Site -> Site -> Handler Mappings. If the Default website is working ok, you can just revert the vdir/application to the parent.