COM Add-in for Excel installer problem

I have developed an Excel Add-in in Visual Studio 2008 for MS Office 2007. My client now wants this Add-in to work on office 2010 also. So, I used Visual Studio 2010 to achieve this requirement. Visual Studio 2010 automatically converted Visual Studio 2008 code to Visual Studio 2010. But the framework 3.5 remained same for this converted project, so I cannot compile this converted code in .Net framework 4.0 (It gives error). Hence, I left the code as is in the 3.5 framework and I created installer for office 2010 in visual studio 2010 & installed. Installation was successful but Add-in was unable to load because load behavior always gets changed from 3 to 2(error: a runtime error occurred while loading of COM-Add-in). So after searching on web I added an entry in the registry through the installer. It is working fine on office 2010, but it is not working on office 2007. I have to keep two different codebases & two different installers for office 2007 & office 2010 although code is exactly the same. I want singl开发者_如何学JAVAe code base & single installer for my Excel Add-in that should work on office 2007 as well as office 2010. Can anyone help me in this? I would be really grateful if anyone could give me a solution at the earliest.