I just did that recently. I was playing with the "net user" command in cmd and never realized that I changed my password.
There is a very easy (and Leagel) way to fix it.
~:What You Will Need:~
1. You windows install dvd.
2. Your brain.
The solution to this problem is very simple. First, let me note that this WILL NOT change default passwords. This will just simply recover from a recent password change.
Ok, first you insert your windows DVD and boot from it.
Now, select "Repair My Computer".
Then select "System Restore"
Now, restore your computer to a point before you had changed your password.
And there you go. You should now be able to log onto your account using your old password.