Try using this:
<system.webServer>
<modules runAllManagedModulesForAllRequests="true" />
...
</system.webServer>
EDIT:
The solution above will work for .NET 3.5 or below.
If you are using .NET 4.0 or above, you might want to try installing IIS7 QFE
Also, this article is worth reading to understand the difference between those two.