Help with dell dimension 1100 please?

I am trying to set up a dell dimension 1100 with an akura monitor but seem to be having difficulty changing the screen resolution the arrow will not move from 640 by 480 pixels and everything on the screen is oversized. I had to reinstall windows originally was on xp home edition but reinstalled xp media centre can anyone help me with this also trying to run with a usb adaptor to connect to router but cant get online? Thanks