try downloading and installing

Updated ati drivers
*reboot*
latest ATI SDK tools
*reboot*
latest cgminer

Have a look at settings other users have had success with on this page
https://en.bitcoin.it/wiki/Mining_Hardware_Comparison

still no luck?  try bfgminer

still not working, idiot-proof solution is BITMINTER.

remember to mess around with your video cards clock settings with http://event.msi.com/vga/afterburner/download.htm