You'll need to work with Dell. They'll want to know the Service Tag and Express Service Code numbers from the bottom of the laptop. Those number will let them figure out a password that will work for you.
And they'll want to make sure you're the legal owner.
there is another way round this problem. There are some good free solutions to discover windows passwords. Obviously you have access to another pc so google "ophcrack" and then download the versions for the OS th laptop has - burn the image to disc then boot up from your burnt CD / DVD and ophcrack will find the passwords for windows.