Vista cannot detect monitors

I have a computer running Vista Home Premium SP2, with an Nvidia 8800 GTX graphics card. Everything has performed fine until a recent update to the Nvidia drivers (or possible DirectX – there is a restore point but I do not know what was installed here).
On boot the screen shows the usual text but overlaid with broken green horizontal lines. These are replaced by white gibberish characters on the next screen and text is unreadable. Text in bios is also corrupted, though generally decipherable.
When (if) the windows logo appears, the lines reappear as blue continuous horizontal lines. If a visible screen is reached, the lines become blue, vertical and in bands and remain visible through the background and overlying icons or screen.
The monitor icon is missing from Device Manager (not hidden either), and a blank “default” monitor appears in screen properties, although some resolution settings work.
Attempts to reinstall the monitor drivers (Viewsonic VX2235wm) get the response that the monitor is not detected. No monitors appear at all in the “Add Hardware” applet in device manager,

I have tried:
(1) totally deleting the Nvidia drivers and software. However, the system still identifies the card as an Nvidia 8800 GTX and installs some sort of driver. I cannot get it to start up with the standard VGA driver. Unless I disable this driver, I will only get a black screen on boot.
(2) Reinstalling nvidia drivers – no change
(3) Different monitor – no change (Acer 19” LCD)
(4) Reinstall monitor drivers – cannot do this as software does not find the monitor
(5) Rebooting in safe mode(s) – necessary to overcome black screen in (1) above – nothing else achieved

I feel that if I can get the monitor to install I might be able to remove this corruption with proper settings. Can you help? Would an upgrade to Windows 7 affect this in any way?