LOL...we did the same thing with a friend's PS3 a few weeks ago. He had it set to 720p at his house, but brought it over to use on a standard-def TV. Luckily we had another PS3 where we would just step through the menus one button press at a time, and got it changed.
If you can wait a little bit, I'll get on my PS3 and give you the button presses to step through and change it back. We learned that each time you turn on the PS3, it seems to default to the games folder, but on the PS3 that we couldn't see anything, we pressed left more than enough times to get all the way to the left folder, then right once to get to the system settings. To make sure we were in the same place, we pressed up quite a few times to make sure we were all the way to the top, then it was easy from there on.
EDIT: Sadaiyappan made my process seem to involved 😛. I wish we knew that, becuase it took a couple of those guys about 20 minutes to get themselves straightened out.