PowerBook G4 running slow after re-installing system

My PowerBook was only getting to the screen with the gray apple at start-up and a Mac Genius recommended erasing and re-installing the system from the disk that came with the computer. I did this successfully and it fixed the start-up problem; however, my computer now runs slow. There's a lag when I click on another window and sometimes the words don't appear on the screen nearly as fast as I type. Is there something I can do about this or is it just age? My computer is about 4 years old.