Just a couple of thoughts. Is your sign-in user account name Administrator? If not, then there is a possibility you didn't create a password for Administrator account itself? In which case, sign-in as Administrator leaving the password field blank. A long shot at that.
How complicated was your password that you don't remember? If you think it wasn't very long or complicated, then you should consider creating Linux based brute force password cracking disc. There are several available for free on the net.
You'll need access to another computer to create a self-booting CD or USB drive. If using a notebook, pressing F2 at start-up will generally bring up boot options for the CD or USB drive.