Why can’t I change my resolution?

Why can’t I change the colour settings past 16 clours and my resolution won’t go higher than 640×480?I have never been able to change my resolution since i changed the monitor but I had the colour up to 32 bit, but then I ran a registry cleaner and now it won’t go past 16 colours.I have an ati adapter and an everdata monitor, and there are a few other glitches on the system but they were there before this happened…HELP ME!!!!