I had this going on since I placed Win 7 on my machine as well (just look at my specs). Even Linux does it when it runs on my machine. And does the same on another screen as well (so it's not the screen).
Never had issues with that, it just lasts a few secs and there are no further problems I could detect afterward (when playing games or using Aero or whatever needs the GPU to work).
Still, I'm mildly curious about this.