IIRC you have a GTX280? That being true you don't want to run GPU3. Download the GPU2 client and give that a go. Think I mentioned this in another thread. GPU3 doesn't play well with GTX2xx cards and the PPD is poor. My GTX260 and GTX275 are on GPU2 which is Core 11.
Be sure and totally remove GPU3 folders and then install GPU2, I hope that works for you....