Nvlddkmkm.sys Error

Hi all.
I've started having grief with graphics.
I got a BSOD with the above error.
Searching seemed to point to my NVidia card. (GeForce 7900GS)
I have a second NVidia card in a second computer (GeForce 8600GTS)

Using the 7900, the computer will boot into safe mode but still has 'blocks' on the desktop.
I've uninstalled etc several times. I usually get the BSOD. It will then get to the welcome screen, screen will go black or it will just freeze.
I put the 8600 in but still get strange graphics at the DELL screen, grey and white blocks.
The computer boots ok with this card but gives me an exclamation mark in device manager stating 'Windows has stopped this device because it has reported problems. (Code 43)'.
It seems to have the correct driver for this card installed so I don't know why it has been stopped.
Graphics are normal though large icons as it's not working as it should.
A second hand 7900 isn't expensive but if it's not the card then it would be pointless buying one.
Any clues?
Cheers

 

Update
I've tried the 7900 in the DELL that the 8600 was in and I get the same results.
BSOD, same error, won't boot into windows.
The only thing that I find strange is why the DELL screen isn't it's usual smooth black background with the 8600 card and why windows is stopping it?

 

Thanks for the reply plodr.
I started some of the methods but as I can't log in to normal windows with the drivers installed I'm struggling.
Something doesn't ring true regarding it being a driver issue.
I thought, how come the DELL screen has corrupt graphics even before any part of windows has loaded.
So I plugged a spare formatted HDD I have into the primary HDD slot and started installing windows.
The DELL screen was still corrupt and the blue default background when loading a fresh install of windows is also corrupt.
This must surely rule out any corrupt files?
I am now installing windows on the spare HDD with the spare graphics card installed.
All graphics are looking normal thus far.
It all points to a faulty card except the dodgy DELL screen when the spare card was plugged in.
Again, no files are loaded at this point so unless my spare card has a problem as well I'm stumped.

 

And a further starnge development.
The installation of windows failed to load on my spare HDD (neither here or there).
So I plugged the original HDD back in leaving the spare video card plugged in.
The DELL screen was normal, windows loaded normally, the graphics card was acting normal.
Device manager said device isworking properly.
The correct driver was listed.
I went to drive C\NVidia etc.
The driver in that folder is the driver for the original 7900 card?
Your confused??
I may just bite the bullet, buy a card and see what occurs.
At least that will answer the question (I hope).
Unless you know different!

 

Update.
Installed 'new' graphics card and all is well (thus far).
For those that get this error and have dodgy graphics, make sure you look at the motherboard screen on boot up.
If the screen is garbled then it's not going to be any driver files causing the problem as they haven't even loaded yet.
It's going to be a hardware issue.