I wonder whether this is another Vista glitch.
Is there a graphics card limitation there or something?

How much memory do you have?

If you have motherboard graphics, the graphics memory is SUBTRACTED from main memory.

The default program directory location for BOINC was also reported to cause a problem for some.

Now, back to that. Many of us change the screensaver to (none) and run it as a very low priority task. That assigns the screensaver percentage to actual crunching. It should back out of the way if you want to do something else of more importance. You can also look at the graphics directly at any time if you want.

If it runs perfectly while not in screensaver mode, why not just do that?

I also set the energysaving feature of the monitor in the control panel to turn itself off after 15-30 minutes or so. That way it can crunch in the dark until I move the mouse to restore the monitor.