WPF Window with AllowsTransparency - bug with behind window

I use a window as a popup, with AllowsTransparency = "True". This popup is shown in the middle of the screen.

The behing wind开发者_开发百科ow, currently show a "loading" animation (this animation is a specific user control).

When the popup window is shown, there is a little bug : whe can see the animation blinking, as if the popup window render a transparency with the behind window animation (but just the animation), but not totally (causing this kind of blink)...

I use windows XP, and I wonder if it's a non-hardware acceleration issue :/