User controls

I have a strange problem, In my windows application, I created a simple user control ( just added a new user control, with NO CODE ) and I built the solution, then I tried to drop this usercontrol onto a simple windows form ( again no other code ) and it complains that The library containing the control is not built or some such, I have been able to successfully create many user controls, so i know it is NOT a programminmg issue, rather something to do with references or my visual studio settings. Does anyone have any ideas as to what i should try ? -S

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