Unable to call a VS2010 class library from a VS2008 windows service

Question

Hi,
I have a windows service, created in VS2008, running as a service. This service is supposed to instantiate a class library created in VS 2010 and execute a method within the assembly. But whenever it tries to instantiate, I get the following error. If I recreate the same assembly in VS 2008, no error is thrown
Could not load file or assembly 'file:///C:\Program Files (x86)\ABC\ABC.Code\ABC.ClientLetter.dll' or one of its dependencies. This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded.
Any suggestion is greatly appreciated.
Thanks