Picked up these instructions from Dell's site, note getting to the first screen mentioned can be tricky. Those computers come loaded with junk so you'll be back to the start again and need to clean it up as the other poster mentioned. Instructions were found here: http://dell.com/support/article/us/en/19/SLN117599/EN
Restore the computer's software to original condition.
An external peripheral is a device that connects to a port on the front, side or back of the computer, like a printer or memory key.
Turn on the computer.
As the computer starts, press < F8 > on the keyboard until the Advanced Boot Options menu appears on the screen.
You must press before the Windows logo appears on the screen. If you press after the Windows logo appears on the screen, the Advanced Boot Options menu will not appear on the screen. If you do not see the Advanced Boot Options menu, restart the computer, and then repeat this step until you see the menu on the screen.
Press the < Down Arrow > on the keyboard to select Repair Your Computer on the Advanced Boot Options menu, and then press < Enter >.
Specify the language settings that you want, and then click Next.
Log in as a user who has administrative credentials, and then click OK.
Click Dell Factory Image Restore.
In the Dell Factory Image Restore window, click Next.
Click to select the Yes, reformat hard drive and restore system software to factory condition check box.
The computer is restored to the default factory configuration.
When the restore operation is completed, click Finish to restart the computer.
The software is now installed as it was when the computer was first received.