Well doing what EricB said is defiantly an option, but do a little research first. Since I don't know exactly what dell model you have, I can't do the searching, but take and search google with the term "dell default password" (not necessary in quotes). you'll come to sites that list the default passwords that most major companies use along with what they use them on. You can try some of these. If all else fails, call dell before you go breaking into the computer with password renew or reinstalling windows, Unless of course you are just going to blank the hard drive anyway.
This site might give a little bit of insight as well. NEOHAPSIS - Peace of Mind Through Integrity and Insight
Get back with some more information on if this works or not and more solutions can be told.