Cannot reference dll in windows service

I've created a windows service in VS 2010 which references the model conveniently placed in a separate project which compiles to a dll. I have no problem referencing this dll in other parts of my solution (web apps), but in my windows services the following symptoms display:

1. Compiler error: The type or namespace %%% cannot be found (are you missing....
2. I add a reference to %%% (directly to a dll copy or to the project, makes no 开发者_如何学Cdifference).
3. The compile error dissapears.
4. I build.
5. The compile error reappears.

Using fw4.

What's up with that?

Edit: I can add and use other dlls ofcourse. Third party and microsoft's.

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Clear out your Debug & Release folders that are created and try doing a full "Rebuild"

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Always check your warning messages.

System.Web.Extensions Assembly cannot be resolved