I made a program that flips a black back surface to a primary surface and has the capability of switching between full screen and windowed mode, and adjust to changing display modes. So basically a game program with no game in it... This is my question: Why is it that in windowed mode I seem to reach a good 500 frames per second and in fullscreen only get about 80?

In fullscreen mode, you are likely being limited by the vertical refresh of your monitor or something similar.

Hope it helps

That would be due to Vertical Synchronization in Fullscreen mode. If you would like to remove this, in your call to flip the buffers, you have to include the flag to disable VSYNC. I think it''s "DDFLIP_NO_VSYNC" or something like that. Look the exact flag up in the SDK.