Direct3D app runs fine if Windows Explorer is running, slows down terribly otherwise

Question

Our Direct3D9 app works perfectly if we fully start Windows, and then launch it. However, if we tweak Windows' startup process so that Explorer is not ran, and just Windows and our application are started, our app slows down horribly. CPU consumption goes to 50%, even when idle (normally, it's near 0).

This can be fixed by starting the Task Manager, starting Explorer and restarting our app. But that's not acceptable; our app must run without Explorer.

Could this be a Direct3D thing? Our app is full screen, Windows version is XP SP3 with all latest updates, and Direct3D version is 9.0.c. If this is not Direct3D related, how could I go about debugging this?

Answer 1

Solved it. Went to Control Panel->UserAccounts->Change the way users log on or off and saw that "Use the Welcome screen" was checked.

Unchecking that solved it. It seems that when the Welcome screen is active, since it uses a different resolution than the desktop's, conflicts ensue. And I didn't check, but it seems that this caused hardware acceleration to be temporarily unavailable and thus software rendering was selected, killing the CPU.