I have tracked my issues down to the following assemblies and I'm now going to start to add them one at a time to see which cuase the issues

<add assembly="System.Data.Linq, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
        <add assembly="System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
        <add assembly="System.Xml.Linq, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089" />
        <add assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
        <add assembly="System.Web.Abstractions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
        <add assembly="System.Data.DataSetExtensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089" />

Okay have narrowed it down as soon as I add

 <add assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />

It all goes potty... I'm still farily new to .NET only a year nnow but why is that the case?

John

Afriad thats not solved the problem see the 1.0.61025.0 version of web.extensions obviously can't work along side the 3.5 version but if the 3.5 version is installed dotnetnuke keels overs as far as I can see this is not a vista specific issue I've now tried it on Server 2003 too. to get DNN to work with the 3.5 version should I recompile to source with the 3.5 reference?
I'm assuming that it is the Client API that is relying on this assembly