Instead of tweaking the task scheduling (which may break your system if done incorrectly) I suggest looking into why your AV is taking that much cpu.

Maybe it's scanning all files? It's no use scanning BMPs i.e. Tweak you AV to scan only "dangerous" filetypes.

It could also be the last update you did isn't quite working properly. Did you check the Symantec site? This may be a known problem.