When you first boot the computer it loads the bios that checks your hdd, dvd ram etc. this information is displayed at boot time.
I gather that not knowing how to get into bios, you are not booting off the cd/dvd and the pc is trying to boot from the hard drive.
You need to enter bios to set the dc/dvd drive as first boot device in boot order screen in order to run the setup from the optical disk.
I don't know which key you need for Dell, but the usual ones are Del & F2. If you watch carefully at boot it's usually displayed at startup "Press ?key to enter bios".
Save any changes in bios settings, after restart it should boot off the cd/dvd allowing you to run setup.