Windows and XBox use two different sets of XNA assemblies. You need to build the library against the Xbox assemblies to use them in an Xbox project.
Create a new class library project, and add the source files from the existing project to this one (add them as links. Add Existing Item, then in the dialog pick the files, on the Add button click the little arrow and select to add them as links).
In the new library, reference the Xbox assemblies, they are at
c:\Program Files\Microsoft XNA\XNA Game Studio\v2.0\References\Xbox 360
So right click on references, select "Add Reference", then in the dialog go to the browse tab, and find the Xbox assemblies at the location I included above.