This is your GPU returning invalid hashes from it's kernel. This is because new drivers are crap for older cards.

- Uninstall AMD APP SDK

- Uninstall AMD Catalyst Install manager: choose "uninstall all software", make sure you have only a VGA driver after restart.

- Manually remove these files not removed by the installer; any other obvious ATI files with the same file date:
  C:\Windows\System32\amdocl64.dll
  C:\Windows\System32\amdoclcl64.dll
  C:\Windows\System32\OpenCL.dll
  C:\Windows\SysWOW64\amdocl.dll
  C:\Windows\SysWOW64\amdoclcl.dll
  C:\Windows\SysWOW64\OpenCL.dll

- Install just the Catalyst 11.11 driver package (no SDK is needed; the AMD APP runtime is included with driver)

- Mine just a bit slower on your 6870 than a 5830 would.

Better:
- use cgminer
- set phatk worksize to 256 and underclock GPU RAM to 300MHz for best power efficiency and higher hashrate