I tried CPU mining both XPM and QRK, found that while doing so, even with one or 2 cores left free (6 core processor), I was still way more likely to have my rig completly lock up and have to be rebooted,  had 3 or 4 hard lockups in a day or 2 of trying to mine either cpu coin, had to hit the reset button each time ... doesn't seem to happen when I use the cpu to mine scrypt based coins (can do roughly 40KH/s on scrypt with 5 out of 6 core). Been debating overclocking the cpu for a bit better returns, but would want to upgrade my cooling before I try.