Look for Display Adapters in Device Manager. If you see ! or ? there, you're missing the driver for it. With only 640 x 480 resolution I agree it sounds like you need it. I went to http://www.emachines.com to see what drivers would be there, but all the models they have listed begin with a LETTER, not a number. Also in your note you neglected to say what version of Windows you have. It would seem that you've learned that the time to address drivers is BEFORE you format the hard drive, not after. In fact most computers provide a way to restore to factory settings. This is much more preferable to formatting and reloading because it takes care of the drivers and any factory installed software as well.
I've just got hold of an old desktop PC for my son, it needed reformatting, and once done. I had a problem with the screen resolution, it was on the lowest default setting,640 x 480 pixels, and when I tried to change it, in screen settings, I was unable to do so. I've been into device manager, and looked to see if I can update the drivers, but I'm sure that it's not showing there, but not knowing the precise name of it, I'm not sure.
I do know that it was ok before I reformatted it, because it's an old work PC that I had.
Can anyone help with this, obviously I need to install a driver, but which one, and how, the PC is an emachines model 390,