Paladin has a hack conflict utility that may help in this regard. go to
Paladin's Place - The Home of SimWardrobe and pick up the conflict utility and run it. I have had similar problems when I installed a lot I downloaded that had a hidden bunch of mods that were not compatible with my installation. So you may want to run
Sims2Pack Clean Installer to see if you have hidden hacks.
Also - I am moving this to general help since it does not treat specifically with Pets EP.