There is probably a graphics card compatibility problem. Make sure you are using the most recent display drivers for your system. See here for more information on graphics card compatibility.
This problem is caused by having hardware anti-aliasing turned on for OpenGL applications. Since Blender uses OpenGL for its 2D display, anti-aliasing will interfere with Blender's operation. Please turn off OpenGL anti-aliasing in your graphics card driver settings to correct this behaviour. See here for more information on graphics card configuration.
Different parts of Blender are dependent on different pieces of computer hardware. A faster CPU and more RAM can help to increase rendering speed, while the speed of the Blender interface, viewports and real-time engine is influenced by the speed of the graphics card. Fast and large hard drives can also speed up work when dealing with large video files.
It appears that Blender versions after 2.23 get slower and slower on ATI cards with the catalyst drivers version 3.10 (or so) and later (4.5 is latest at the moment). The quick fix seems to be to use older drivers (at least for the opengl driver).
The Blender Foundation was sent 5 ATI graphic cards from ATI to look for a soution to this problem. For now a possible fix for you system can be found here.