I haven't used Process Tamer in a while, but I'm pretty sure you could accomplish this with it. Process Tamer has an option for making itself and Task Manager high priority.
Then just adjust the options so that anything above 1% or something will lower the process. Something like that ought to do it for you.
Kinda silly though, as it doesn't help performance - and could in fact hurt performance. Consider a well-written
application that does number crunching, and has set it's crunching-thread to BELOWNORMAL priority. By setting all
process/thread priorities to BELOWNORMAL, all applications will be battling at the same priority level, rather than running normally and giving the heavy number-crunching thread less attention. This could
result in GUIs performing sluggishly etc.
It's best keeping normal threads at normal priority, and only assigning lower priority to those ill-behaving processes that gobble up CPU and don't lower their own priorities.