If you want to set priority when launching a process you could use the built-in START command:
START ["title"] [/Dpath] [/I] [/MIN] [/MAX] [/SEPARATE | /SHARED]
[/LOW | /NORMAL | /HIGH | /REALTIME | /ABOVENORMAL | /BELOWNORMAL]
[/WAIT] [/B] [command/program] [parameters]
Use the low through belownormal options to set priority of the launched command/program. Seems like the most straightforward solution. No downloads or script writing. The other solutions probably work on already running procs though.