When trying to debug a WCF ( windows communication foundation) .NET 3.5 framework library, you might encounter a problem where Visual Studio (VS 2008) refuses to run or debug the WCF web service.

the problem:

The error you get from the WCF test client when trying to debug the WCF service is this – "WCF Test Client: Unrecognized option ‘targetclientversion’ specified"

The solution:

1) Close Visual Studio

2) Install the latest version of Windows SDK for Windows Server 2008 and .NET Framework 3.5

3)  open Visual Studio again

This solved the problem for me.