If you get this error message when running a c# application in Visual Studio 2010 by pressing F5 (Start Debugging) then the chances are you have multiple browsers installed and this has caused problems with the debugger.
Microsoft Visual Studio
Attaching the Script debugger to process ‘ iexplore.exe’ on machine ‘machinename’ failed. A debugger is already attached.
Debugging still works but you have to cancel the annoying message in Visual Studio every time.
To get rid of the error message simple close all browsers (leave VS 2010 running if you like) and then in a command window with admin privilege, simply enter the following command then continue debugging as you were before the problem started. Simple!
regsvr32.exe "%ProgramFiles(x86)%\Common Files\Microsoft Shared\VS7Debug\msdbg2.dll"