The Dell Dimension 8300 series of computers includes a restore partition built into them that allows you to restore the computers to their original factory condition. If you need to reset your computer to the condition it was when it arrived from Dell, you can do so, but be sure to back up your data first. The factory restoration process completely erases all data on the computer and overwrites it with a new system image.
Restart or turn on your computer. Press the "F8" key several times as it starts up until you see the Advanced Boot Options screen.
Select the "Repair Your Computer" option and then click on the "Next" button.
Log in to the console with a Windows user account that has administrative permissions, click "OK" and then click on the "Dell Factory Image Restore" option.
Click "Next," click the "Yes, reformat hard drive and restore system software to factory condition" box and click the "Next" button.
Wait while the computer is restored to its original configuration. Once the process finishes, you can log in and start using it again.