I was having the same problem with my nVidia 8800GTS-OC. I installed Windows 7 last night and got garbage over the monitor screen and from time to time even blue-screens and crashes. This morning I tested the PC on my TV (which has HDMI) and it runs very fine.
Unfortunately I will have to change my monitor, as it only has VGA input.
Edited later: not really, cause I used my TV, installed new drivers from nVidia, connected the monitor through the VGA-DMI adapter as a second monitor and it seems to work fine now.