Is it a Windows computer? You likely have to reformat it and re-install Windows on it. If you don't have the original cd for the recovery version, there should be a recovery partition that contains it. Are you sure it is not the same password as whatever your current computer has? I don't change my password from computer to computer as long as it has not been compromised or shared with anyone else (and it never has been). If your computer has a "forgot password" option and it's linked to a Microsoft account or you set up security questions for local access, you may be able to recover and change the password. I don't remember if Windows has another way to reset a user password unless you enable the hidden administrator account (which usually has no default password - you must set it), but it can only be enabled if you are already logged into a Windows account. I did find this, though:
