I have the same problem with the "high quality graphics" setting on, even with the lowest possible resolution. With the 1024* resolution it already becomes impossible to move the mouse.
Turning it to "low quality" makes the problem go away and I can even play on higher resolutions now.
I have a 2,3 GHz "Pentium 4",
with 1GB of ram
and a 256MB, 128bit "Radeon 9250"