Now it depends on what OS you are running as you do it slightly differently with the two. I am assuming you have vista but will include the xp method aswell.
1. Start off shutting down your computer and unplugging all external devices eg: printers etc
2. When you turn on and you see the bios screen press F8 until you see the advanced boot options menu you must do all this before the windows screen appears. if it doesn't work just shutdown and try again.
3. navigate with arrow keys to the repair your computer option, then select the appropriate language
4. log on to the system with your account (assuming that you are an administrator)
5. select Dell Factory image restore and press next
6. Then tick the box that says Yes reformat the hard drive and restore system software to factory condition and then click next
7. After a while the process will complete then press finish and the system will reboot.
I hope that was helpful, if you don't have vista but have 7 or XP just say and i will post the appropriate info