It sounds like you may have knackered your registry and IRQ settings a bit.
When you removed your old ATI drivers, did you have the ATI card installed when you did this? If not, they won't have uninstalled properly from the Registry, and your PC may still be looking for some form of ATI card.
Before you try the following, obtain the Driver Cleaner Pro software from
www.guru3d.com It will help clear out any left over driver files not necessary.
Run the driver Cleaner software, and remove all reference to your ATI drivers from the PC. Reboot the PC and see if this fixes the problem, if not, follow the next solution.
Leaving your Nvidia card in, uninstall the NVidia driver suite completely, shut the machine down and then remove the card. Replace your ATI card, reboot the PC and see if the ATI card redetects itself and installs. If it does this fine, then it means the uninstall of ATI software didn't uninstall the card, just the software. Wait for your PC to install the card, then reinstall the ATI software and drivers. Reboot the machine.
When rebooted, then completely remove the ATI drivers and software again. Then run the Driver Cleaner Pro software to completely remove all the ATI drivers etc this time around. THis should hopefully rectify the Registry and make it work properly.
After the software and driver removal, shut the machine down. Replace the ATI with the NVidia and reboot the machine. Then run the Nvidia software and reinstall your drivers. Hopefully now you shouldn't have a problem.
If you still have a problem, then you may have knackered your machine, and could need a rebuild to fix it.
Unless someone else has a solution, I won't recommend attempting to go into the registry unless you know what you are doing.
You have learnt something here, Learn to Read the Installation Instructions before attempting the install of new hardware. It is there for a reason, unless you know what you are doing.