Sprites being wiped-out

I'm not using DirectX, although I've created mouse controlled sprites using mainly API functions in Visual Basic. My sprites are being wiped out when another form is moved over the drawing area (picture box). What's wierd is that the bitmap background remains. I though I was blitting the sprites on to the background bitmap, so why or how the sprites are being wiped-out is pretty bizzare? These are MDI forms. Any help? Thanks.

you have to set the property AutoRedraw to True!

I tried that and several things, but are you referring to the form's autoredraw property as I haven't tried that yet. I'll give that a shot. Thanks.