alright so if you have a CD for your operating system then just during boot time, boot from the CD, and there will be a an easy-to-follow self explanatory wizard to guide you through. Depending on your OS, procedure will vary.
Reboot the computer, right after your see the Dell BIOS spalsh screen, hold down the control key and tap the F11 key. It should boot the Dell System Recovery program.
Just keep tapping the F11 by holding the CTRL key if its XP and select dell image factory restore...But if in case if vista is installed on the system u have keep tapping the F8 while booting and the select the option Repair ur computer...select english...then system password and finally select the last option in the list Dell image factory restore.....
if this doesnt work u have to reinstall with the OS disc by booting from the disc while start up.