remove all XNA dependencies from all projects of your solution
add MonoGame & Lidgren project to your solution
add "MonoGame.Framework.Windows", as dependency to your project (if you use farseerphysics, add monogame also there)
remove "Content" from your Project (on filesystem too, restart VS2010)
add a "Content" folder in your Project using VisualStudio
add all Content Stuff to this folder
Edit each content file for "Content" and "copy always"