I'm at the end of my Googling rope and still having the same problem.  I downloaded the source code for a module I'm using in my .NET application so that I could do some customization and recompile.  Once I figured out how to use MSBuild to compile the DLL (I don't have VS) I copied the resultant DLL to my applications bin directory, overwriting the old DLL of the same name.  Since then, I've learned that doing this and refreshing the page isn't going to be enough because of the caching of DLLs in GAC.  So here's what I've tried:

1. Used iisreset.exe to restart the app server (IIS).
2. Updated the assembly file to ensure the DLL got a new version number. (recompiled/deployed/reset IIS)
3. Tried using gacutil.exe but don't see anything listed in the GAC related to this DLL.

I've checked everywhere to make sure that MSBuild is not recompiling old code.  As far as the code, so far I've only changed a pop up error message so I could tell when the new version was in effect, and of course the old error is still displayed.  If any code would be useful, let me know and I'll post it but it didn't seem so much like a coding issue to me.

Thanks in advance!