My win7 Ultimate x64 OS sometimes flickers after hitting the desktop when start/restart.
This sometimes means (I found out) that taking screen shots will fail when pasting them into MS Paint.
I also have a NVidia card on that rig (256 MB), its been so long since I booted into that OS I don't remember what drivers I'm using.
Try resetting your video card/check its all the way down & check the power connector.
Consider a new updated driver.
Is it a overclocked edition?