What you are experiencing is a security feature in Windows 2000+. If one
application uses SetWindowsHookEx() to hook another application run by a
different user, the hook fails, even if the target process is visible and on
the same desktop. I just tried this with Notepad, it works fine when I start
it as the same user, and doesn't work when I use Run As to start it as a
different user(tried Administrator and Power User). I am logged in using an
account that is a member of the Administrators group. SetWindowsHookEx()
returns 0 in these cases and GetLastError() returns 5, Access is denied,
ERROR_ACCESS_DENIED. The hook procedure is in a C++ DLL.
Also, in Vista and later, SendMessage can only send messages to processes
with the same privilege or lower, otherwise it fails with error access
denied. See here:
SendMessage Function
http://msdn2.microsoft.com/en-us/library/ms644950.aspx
Finally, while on the same subject GUI and security, staring with Vista;
services run in their own terminal services session for security reasons, so
services that create a hidden window to let other applications talk to it
using SendMessage(To configure the service, for example) will be blocked
from receiving such messages in Vista and after. You have to use another
method like named pipes. See here:
AC: Session 0 Isolation
http://msdn2.microsoft.com/en-us/library/bb756986.aspx
