Place a voice call to Dell support with verification of ownership.  If the system is out of warranty, have a credit card handy for the charge.

Unfortunately, Dell doesn't have any customer accessible tools to reset an administrator password. Our best bet would be to Contact Dell GHN (Get Help Now) chat technical support Monday through Friday. Click the blue "Get Help Now" on the right to start a private live chat session. Share the private Service Tag with them so that they can verify the ownership and warranty status. This will also generate a unique Technical Support case for your unique Service Tag. If already out of warranty, click here for the Dell out of warranty offerings.
We'll be able to get that password issue resolved for you!