|Did you make a recovery disk when you got the computer?|
In which case you boot from the recovery disk in the Cd/DVD drive and follow the on screen instructions.
Or alternatively, if you made a disk image you can press F2 when the computer boots in reinstall the disk image you made.
If you have neither a recovery disk or a disk image you are pretty much screwed. You could try contacting Dell and see if they will supply you with a recovery disk but you will have to pay for it.
Using the recovery disk will delete all data on the computer so you will need to backup any data that you want to keep.