This is really frustrating.  I've sent two projects two a coworker.  One is a website and the other gets built into a DLL.  On my computer I simply copy the DLL and PDB files into the bin folder of the website.  I immediately have access to the intellisense and can step through the DLL code in debug mode.

My coworker copied the files to the same place on his computer, loaded the solutions for both projects, can build them fine, copied the dll into the website's bin folder, it runs fine, however no intellisense is available.  On top of that, when he tries to step through the code, a file browser pops up asking him where it is.  The class names and everything are rendered in Visual Studio as normal black text, and have no info on the mouseover.

We tried creating two new projects on his computer and adding the existing code to them, but VS still can't see the code for the other project.  What I don't understand is that it runs perfectly.  It has the pdb file, so what is wrong with it?
 

We found a solution, I figured I'd post it in case anyone else runs into this:

Go to the tool menu and select import and export settings, then select reset all settings.  We are not sure what the problem was, but this fixed it.

After we did this, Visual Studio was able to figure out where the source code was and everything now works properly.
 

Well I got the problem aswell with the prompt that no source code was available when trying to debug a page with ajax control toolkit controls and I was able to fix it with your above solution. Thanks for posting that. Hope It keeps workin for me now. Have no idea what else might be the problem.

 Maybe a reinstall of VWD and the Ajax Control Toolkit...