asp.net 3.5 not wark in net freamework4

hi i need help
my iis7.5 and wen i change iis to framework4 the website is work did not work with asp.net3.5 website in windows7 only asp.net4 work the error is

HTTP Error 500.19 - Internal Server Error

The requested page cannot be accessed because the related configuration data for the page is invalid.

and wen i change iis to framework2 the website is work
please help me