I'm not exactly sure what your problem is. But it sounds like it may have something to do with your video card. You could try running Osu! in a lower version of DirectX or turn off vSync or something, but I don't guarantee anything will work.
Create an Osu! shortcut (anywhere it doesn't matter) and right click it and press properties. Under the Target: field go to the very end and put a space after the last quote character then type "-dxlevel 80" without quotes. If that doesn't work try "-novsync" without quotes. If niether of those work, I'm out of ideas, sorry
Oh, and you could also check if you have the latest video card drivers.