Idle/low priority non-focused processes, except system processes, though idling most of them is ok - it seems only csrss.exe and System need to remain normal or higher unless you want UI slowdowns and audio/video dropouts. Also as groups: when any of X, Y, or Z are focused, all are non-idle.
Each process can have its focused and unfocused priority set; you could have non-focused at a higher priority. You need to be able to set all process priorities permanently. Prio http://www.prnwatch.com/download.html
helps but doesn't do system processes.
See also suspending processeshttp://www.donationc...ex.php?topic=16746.0