Some of these machines have a hidden partition with the factory image installed. I think if you power it on then hit CTRL & F11 together when the DELL screen appears it loads the restore app.

You can check this by logging on as the guest user, looking in disk management, and see if there is another partition.

Worst case scenario, I used an app called ophcrack to find the admin password, although it was a while ago. I just googled it and it's still around. You may want to check it out if all else fails. (create a bootable usb stick and boot from the app)