Chipset drivers may be required, depending on what Win 7 offers, for things on the motherboard. They handle many of the basic operations and are a good starting point. Win 7 will sometimes install generic drivers that are not as efficient as specific drivers.
What Nvidia card is it, and Nvidia is very good about keeping drivers available. Just download and run the .exe file. Video drivers are changed all the time, even through Windows Update. Did the Dell person remove the ATI drivers? If not there may be a way to do that now, let us know.