Marcus,
not sure if this will help at all???
but i have found that with vista, i don't uninstall current gfx drivers, d/l the new drivers to a seperate d/l folder, then run the d/l'd file, follow the on screen prompts, use a registry cleaner to clean up and finally restart pc. this works for me every time as i had what appears to be the exact same problem you're getting.
so exact, i even took my 4870 back to the shop 3 times for a replacement

, eventually i put the original 9800gt back in and let it find the drivers automatically with windows update, then just d/l'd and installed new drivers.