How to restore display in system preferences

I have a 2009 iMac 24" screen running Leopard 10.5.8; using My Book Essential 2TB as a backup. I just went into the iMac’s system preferences to dim the screen brightness and by mistake double clicked ‘some setting’ and now the display’s fonts are 1/3 larger than they had been. I think the usual resolution setting had been 1024 by 768 but when I clicked on that it didn’t return to normal. My question: how do I access the previous display settings on the backup drive and restore them to what they were earlier today or yesterday?

Thanks for your help. 

I took it to the genius bar today. What I did was to change the resolution to 1024X768 from the original 1600 X 1000. The guy changed it back. Boom boom bam bam 5 minutes later I’m walking out the door.