If you have a recovery CD or a recovery partition on the laptop, then running either recovery will put the laptop back to the way it was from the factory. It will automatically reformat and load everything for you.
If you have a recovery partition, you need to do the following:
The key combination is CRTL + F11 pushing the keys at the same time at the the dell screen during post. Once you push them then you want to release them. It should go into the recovery partition so you can do a system restore.
If you have recovery CD's, then you will most likely have to set the bios up to boot from CD. I would try the CTRL + F11 method first.
I assume that you are having no other issues other than wanting to clean the computer of all personnal data.
Please let us know how you make out or have any more questions.