First, make sure you have a driver installed correctly, one which includes OpenCL. I would recommend the ATI Catalyst 11.6 full driver package as the most problem-free. Use the control panel->programs and features, and uninstall ATI drivers currently on the system (pick "remove ALL ATI software" when prompted). Restart. Install 11.6. Restart.

Then I would recommend the Phoenix miner. Extract it to it's own directory, and save the following to a new miner.bat file in that directory:

phoenix.exe -v -u http://workername:password@us.ozco.in:8332/ -k phatk2 VECTORS AGGRESSION=6 FASTLOOP=True WORKSIZE=256 PLATFORM=0 DEVICE=0

Put in your user/pass and the pool URL that you are using. Note that on most pools, you don't use your main user name, you create a second username called a "worker".

These options give you a miner that will work well in the background while you use your computer. Run the miner.bat file.

If it still can't open the OpenCL device, run just this command, it will print a list of your device numbers, so you can make sure your video card is device 0:

phoenix.exe -v -u http://nothing:8332/ -k phatk2

That should get you mining, you can start tweaking things from there.