How can I redo my computer to when it was first purchased?

BACK UP ALL DATA YOU WANT TO KEEP
Your Mac was shipped with 2 DVDs one containing the OS the other has Ilife and some other programs. Boot from the OS disk by holding down the "C" key and booting with the disk in the optical drive.
Tun Disk Utility, and format or erase the disk do not change the file system type. Reboot while holding down the "C" key and Install the OS then reboot and install the other applications from the second disk. From here just follow the install routine, it guides you through the process then reboot and install the programs from the second disk.
*** During the install process click customise and install "Rosetta" if you need to run applications built for the Power CPU (Non Intel Macs)

Try first repair permissions. After every software update, and about every 2 weeks: close all apps and log totally off. Log on, go in Finder, Applications. Utilities, Disk Utility. After the message -getting disk information- select volume (below the hard drive name -upper left corner). Just highlight it. Now look to the lower two things are there near the middle, verify permissions, repair permissions. Click repair permissions

Remember running disk utility from within a user is not as effective as running it from boot, it can not fix files it dose not have permissions to fix. To run it from boot start as if you were going to install the OS and select Utilities- disk Utilities on the screen after you select the language reboot after you run the utility.