I would perform a factory reset on the phone. To do this you need to go into recovery mode. First, pull the battery out and replace. Now, hold Volume UP + Home + Power button in that order until the Galaxy S logo appears then release all three buttons. The phone should boot into recovery. Use the volume buttons to scroll up and down and the Home button to select.
Scroll to Wipe data/Factory reset and press Home then confirm. Do the same for Wipe Cache partition. Once done, select Reboot phone.