Interop or VSTO ?
for VSTO is possible with VS 2008 and.NET 3.5
look at this picture http://blogs.msdn.com/b/jensha/archive/2010/04/28/vsto-versionen-erkl-rt.aspx
for Interop change references to office 14, excel 14 and clean/recompile
refer this link
http://blogs.msdn.com/b/vsto/
http://blogs.msdn.com/b/mshneer/archive/2008/04/24/deploying-your-vsto-add-in-to-all-users-part-iii.aspx