IIS 2003 server two web sites with different frameworks

Hello,
I have a Windows 2003 server with two web sites: one of them works with framework 4 and the other one works with framework 2.
I0vre created two different application pool one for each website but ony one of them works.
In fact, doing IIS reset and browsing for the web site with framework 4, it works but the other website gives Server unavailable.
Then, doing IIS reset and browsing the website with framework 2, it works but the other one with framework 4 does not work.

In the event viewer there is an error:
It is not possible to run two different versions of ASP.NET in the same IIS process. Please use the IIS Administration Tool to reconfigure your server to run the application in a separate process.

But there are 2 different process with two different frameworks!!

Couuld it depens that the installation order on server? In fact, I'm not sure if IIS was installed before or after frameworks.

Thanks