Either boot into safe mode and see if the 'default' administrator account is still left unpassworded, or make yourself a disk using something like this.
I have the Trinity Rescue Kit set to boot from my USB pendrive. TRK has a winpass script to reset the administrator password.
Trinity Rescue Kit or TRK is a free live Linux distribution that aims specifically at recovery and repair operations on Windows machines, but is equally usable for Linux recovery issues.
It is possible to boot TRK in three different ways:
-as a bootable CD which you can burn yourself from a downloadable isofile
-from a USB stick/disk (optionally also a fixed disk), installable from Windows or from the bootable TRK cd
-from network over PXE, which requires some modifications on your local network.
TRK is a complete commandline based distribution, apart from a few tools like qtparted, links, partition image and midnight commander