1) With a computer that new, I assume you have Windows 7. Did you install the game according to the large-print instructions at the top of the second post
in this thread
2) Do you have the "high-quality" (LOL) movie patch installed?
3) Check your graphics drivers control panel and see if 640x480 is among the supported resolutions (it should be).
4) Mess around with the different scaling options in said control panel and see if that's the problem.