Hi schrecengost,
I found a solution for me.
I deinstalled Infragistics, Ajax Extensions and IIS-Server. Then I installed again IIS, Ajax-Extensions and Infragistics and now it works.
Perhaps you can try first to register ASP.NET again in IIS with the following command
aspnet_regiis -i
I had to do this after the new Installation of IIS. You can try this before the Deinstallation.
Best wishes.
Jürgen