My DirectX Prog. Locks when using Ctrl+Tab!

Hi!

I have made a simple fullscreen DirectX program!

I write directly in the back surface, and there fore I of cause uses the Lock and Unlock func.

It seems to be the fact that I uses these functions that corses the program to lock up when I press Ctrl+Tab or mulitask in an other way.

I do asure that the program is activ before locking the surface. (using the WM_ACTIVATE message)

If somebody could give me a hint of what I could be doing wrong, I would be wery greatfull.

I'm using VC++ 6.0 and DirectX 7.0 SDK.