Have you made sure you have the latest version of both DirectX and your video card drivers? It may seem odd but a lot of programs you wouldn't think would care; actually will have major problems with some video drivers and hardware. For instance Paint shop pro will not install at all on some systems with a dual core AMD Athlon64 AND the latest NVidia forceware video drivers, so you have to install the beta drivers for it to work. I know because it happened to me. :) C5 is pretty picky too from what I can tell. If a full reinstall doesn't help check the drivers and such.