I was also having problems with my 7900gt. Everytime I came out of sleep mode the screen would get all scrambled. Reinstalled microsft drivers and all is well. Havent tried to run any games yet though.
there is definitly something up with the power modes and nvidia drivers. Im just happy the microsoft drivers are working right now.
I've got a 7900gt and I'm running Vista x64 also.
The screen does indeed scramble. As I was unfamiliar with the settings of Vista having just installed it, I left the power settings set to defaults (sleeps after an hour.)
When I came back to my machine to wake it up, the screen pops on but there's scrambling as you put it. The funny thing is that the system doesn't lock, but you can't see the screen - just like lines diagonally across the screen and a browny type colour. I've
changed the power settings (so the thing doesnt go to sleep) but one thing I have noticed is that this happens when I boot into CS: Source - tried everything I could from uninstalling the drivers and adding the vista ones back and visa versa, still no joy.
If you try and tab out of the game, you get the same scrambling but the colour of the theme you have vista running in is apparent. Games such as WoW, Warcraft 3 and the likes all don't have this issue.
With WoW I have noticed a weird bug, where that after you've been playing a while, randomly - the fps drops about 1/3. Strangely enough, minimising the app and restoring it again solves this.
I'm guessing it is indeed a problem with the Nvidia drivers, as other games seem to perform fine bar the odd glitch in one or two.