If this is a BIOS level password, as in it prompts you for a password even before Windows starts loading, then you will need to contact Dell. Once you supply sufficient information to prove you are the rightful owner of the unit, they will tell you how to deal with the password.
did you try to login via safe mode or use Windows Installation CD to help you?
Also, u can use some tools like Windows Password Recovery Tool, Ophcrack, PCLogin, etc. to help you create a password reset disk to get out of problem quickly!