You go into Boot Menu, and set the - first boot device to CDROM.
The computer will then read what is in the optical drive, first.
Computer off, press the Power On button. Watch closely for the blue dell logo.
Looking at the bottom of the monitor screen when the blue Dell logo comes up, you will see - F2 = Setup.
At the top right of the screen you will see F12 = Boot Menu.
This screen goes by F-A-S-T!
As SOON as you see the blue Dell logo press the F12 key.
If you miss this small window of opportunity, just wait for Windows to fully load, then click on Start, Turn Off Computer, and Restart.
If Windows is not loading, then press the CTRL, ALT, and DELete keys at the SAME TIME. (Not shouting)
This will restart your computer so you can try again.
Upon entering Boot Menu you cannot use your mouse anymore, you have to use the keyboard.
The Boot Device Menu has the devices listed, and each device has a number next to it.
At the bottom of the menu, type the number that is next to CDROM, then press the Enter key.
This is Dell Support, and the main page for the Dell Dimension 4700 Service Manual,http://support.dell.com/support/edocs/systems/dim4700/sm/index.htm
Left-click on - Entering System Setup
Should you have any questions concerning this, simply state so in a Comment.