Thanks for your replies and useful tips Roger and Pete!

Pete, I am still a bit confused. Are you saying .NET 4 does not have all the dlls from the previous versions? Thanks in advance!

Hi All, I am new to .NET. I recently developed a windows service using VC#. During installation of this windows service I point to .NET 4.0(I am talking about the "installUtil". Not sure if this makes any difference.) Whenever there is .NET 3.5 and .NET 4.0 present in the system on which it runs my service runs perfectly fine. But whenever .NET 3.5 is not installed but only .NET 4.0 is present my service fails to run and it gives me the following error: Could not load file or assembly 'System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' or one of its dependencies. Does not .NET 4.0 contain everything that .NET 3.5 contains and more? Can anyone please help me with this situation? I definitely cannot ask everyone using my windows service to install .NET 3.5 and 4.0 on their systems. Can someone please provide a solution? Thanks in advance! Regards, Lydia