Unrecognized message 0xC03B received in VS 6.0 application

I have an application written using VS 6.0 VC++. This app has been running
fine on Windows XP and 2K for hears. When I run it on Windows Vista, the
main thread receives a message 0xC03B constantly, such that my app hogs the
CPU even when idle.

I figured out that if I treat this message like a WM_PAINT or mouse move
message, and do not perform idle processing when it is received, everything
is fine. If I do that, I don't get the message constantly and CPU usage goes
to 0% when the task is idle.

But I want to know what the message means. 3B is not devined in winuser.h.
Does anyone know what this message means?

Thank you.