When your Dell Dimension 8400 is first shipped from the factory, it will run at peak efficiency and speed. Over time you will likely notice a serious degradation in performance caused by conflicting programs, unnecessary registry entries and even virus infections. Running Dell's built-in restore feature will completely wipe out all of your current data and restore the system back to its original factory configuration, which will fix any existing problems.
Save any documents or installation files you will need after restoring the Dell 8400 computer to an external device such as a rewritable disc or USB drive. Unplug the Dell computer's network cable and any peripheral devices such as a scanner or fax machine.
Bring up the Start menu and choose the "Restart" option. Tap the Dell 8400s "Ctrl" and "F11" keys when the blue Dell bar pops up at the top end of your screen.
Wait for the system restore menu to appear and then click the "Restore" option. Choose "Confirm" to start formatting the Dell Dimension 8400s hard drive.
Click the "Finish" option after the 8400s format process finishes running. Click "Next" and then select "Yes." Click "OK" and wait for the Dell computer to restart.
Click "Next." Follow the Windows installation prompts on the screen to choose your login name and set up your Internet connection. Connect the network cable and any other peripheral cables back to the 8400.
Use the installation discs that came with your external hardware, such as a printer or router, to install the hardware's driver software.
You may be able to resolve some issues without completely restoring the Dell Dimension 8400 back to its original factory settings. Instead you can use the Windows Restore feature to roll back recently installed driver software or registry changes. If you are using the original operating system included with the 8400, navigate to "All Programs," "Accessories," "System Tools" and then "System Restore." Follow the on-screen instructions to restore to a previous date. If you are using Windows 7, you can reach Windows System restore instead by navigating to the "Control Panel" and choosing "System and Security." Click "System," "System Protection" and then "System Restore."