A petition isn't going to do jack squat. Microsoft is going to put XP out to pasture and make everyone move to Vista if they want to continue using Windows. That means more money for them. A petition doesn't. It's a waste of time.
You want to really make a difference? Stop using Windows. Get a Mac. Tell all your friends and family to do the same. Or explore alternative OSes like Linux or FreeBSD, both of which don't cost a penny and are fun to learn.
My girlfriend went into her first Apple store today and was immediately in-love. She's never owned a Mac but after seeing them and all the different options, she's upset I let her buy a Windows laptop a few months ago.