eM Client does not start due to .Net initialization error

I have just installed eM client and when I try to start it, the following error message appears:
.NET Framework Initialization Error: Unable to find a version of the runtime to run this application.
How do I fix it?
I am running XP with .NET Framwork 4 installed

unfortunately, .NET Framework 4 does not necessarily contain previous versions of the framework. Please install Microsoft .NET 3.5 SP1 package from the following location http://www.microsoft.com/downloads/de… and let us know if that helps.