If your motherboard has on-board ATi graphics, it should be turned off in the BIOS and/or disabled in the device manager, then uninstall the support files for the on-board GFX only and reboot. Install the nVidia drivers you get off their website because they are the best. Microsoft update has been sending out a lot of flawed driver "updates" for nVidia cards of late, so be sure to check the vendor of the driver file you have and are using. if it is a Microsoft driver, roll it back to the previous driver then delete the driver file for Microsoft's "fix" off the hard disk. then make sure you choose the "Do not remind me about this update" option in the windows update page that opens when you either launch Microsoft update in internet explorer or in the Security Center version of it.
Edit: You may have to check the Automatic Updates logfile to see if the nVidia update has been performed. Sometimes the updated files will integrate with the nVidia ones and not report the correct vendor in the graphics card properties or device manager.
Another note: Microsoft's driver files seem to make the cards from nVidia run hotter as well. I have had a few older and newer ones come to early deaths at the hands of poorly designed drivers. (and no, I do not overclock those cards - at least not without proper cooling).
Best of luck!
Edit: on the topic of failures to start, if in XP, you can run system recovery or in safe mode and open the command prompt. Then get your XP cd and put it into the drive. Go back to the command prompt and type in this:/sfc scannow
This will scan for corrupted windows files and then extract what files it can find to replace them from the XP cd. Once finished, reboot. Not sure if this trick will work in Vista or 7. I suppose it is worth a try as a last ditch effort to stave off the reformat gremlin. Of corpse, you may also want to run a full scandisk (or equivalent) to try and recover bad sectors on the hard disk.