Depending on your system you need to leave a thread or two free to be able to do other things while mining.
Try only 3 cores that is what I do on mine.
That way I can do everything and still mine fine.
How are people mining at full and being able to use their system? I'm running an I-5 running at 4.4ghz with 12 gigs of ram. OS windows 7 64 bit ultimate. Running av-2 w/ 1024 per thread (that's the max supported anyway per thread). If I run all 4 cores my system cannot be used for anything else.