Try Process Hacker
. It is a free program like Process Explorer
, except it is faster and much more powerful. After starting Process Hacker, find the process whose priority you want to change, right-click it, and set the priority as desired. Then, right-click the process again, go to "Priority" and click "Save for xxxx.exe." Thereafter, whenever that program is started, Process Hacker will automatically set the priority to what you saved. However, I believe that you'll have to put Process Hacker in startup and always leave it running for it to accomplish this task.
Try searching on Softpedia for "priority"
. In those search results, I saw a couple of programs that would be much lighter that may also work. Most of the programs in those search results won't do what you need, so you'll have to sift through the listing.
FWIW, I have written a simple Task Manager (that makes the [Pause/Break] key on the keyboard useful) that I could add this feature to, if you would like (you won't find it on the Internet, though).