Cheers Smooth! It's an i7-4770, standard clocks and all, running 6 threads, above normal priority.
Why don't you run all threads/cores but in low priority? Windows can manage just fine the other jobs that way, and you can have the max hashrate too.
I even play Left 4 Dead 2 without stop mining. The hashrate drops from 850 h/s to 550 while playing but that's all, I have to do nothing else.
These days, that box is primarily a GPU mining rig, and the current GPU work is actually very sensitive to any moderate CPU usage. Optimal performance (giving priority to current
GPU work), was found by setting affinity to have 2 dedicated threads for the GPU miner. I can then do whatever on the other 6 threads, that the GPUs remain at full performance.