Well I'm just guesing, but there is no aspnet_regiis.exe in the Framework\v3.5 folder probably because 3.5 is really just 2.0 with some additional dlls. So my guess is you would have to run it from the v2.0.50727 folder. 

Hope it helps,

Joe

cd into the folder C:\Windows\Microsoft.NET\Framework\v2.0.50727 (aspnet_regiis.exe lives there) and run the command. google aspnet_regiis for syntax for the command.

I have no idea if thats going to solve the problem as I have no idea what error is happening, your original post had very little information except you point to some lines in Web.config

Hope it helps,

Joe

As long as you installed .NET 3.5 sp1 and you see it in add/remove programs it should work. When you have 1.1 .NET and 2.0 .NET on the same machine you will see a dropdown in the IIS proprties to choose 1.1 or 2.0 but you won't see an option for 3.0 or 3.5 because they are really just supersets of 2.0.

If you give more detail about the error maybe I could help more effectively. I'm heading out for a little while but will be back later.

Best,

Joe