Of your XP system is that unstable after the SP1 update, then it isn't SP1, it's configuration borked by user error. Not being rude, just honest. If the majority of us are working merrily along with SP1, and have been for some time without any sort of hitch (ever), then that may mean something.
I agree with l00p here. But on a different scale. You can either learn to apply security and fix incorrect previous settings (your computer breaks because the settings are insecure), or a simply not worry about incorrect OS settings that you currently have and disregard security. To me, it isn't really a choice. It's similar to saying "you can choose to find a cure to your disease and fix your lifestyle, or choose to ignore all of it and jump off a cliff to end it all"
it has not affected me even a bit but has helped me and my pc to get better