Create a SystemRescueCD boot CD
You'll need to download SystemRescueCD from the website from another computer. It's pretty hefty at about 350MB so it might take a while to finish. Burn the ISO image to a CD using your favorite disc burning software. Be sure to burn using the "burn disc image" or "burn image" option instead of the regular data disc method. Once it's done, put the CD into the drive on your computer then boot from it.
Reset the password
You'll boot to a terminal. This is where the magic comes in. First, mount the partition that Windows is installed in.
# mkdir /mnt/Windows
# ntfs-3g /dev/sda2 /mnt/Windows
Windows XP/Vista should be pretty straight forward. Windows 7 usually has at least two partitions. The first is a hidden system reserved partition, while the second is the actual partition where Windows is installed. Replace /dev/sda2 with the appropriate device on your system. Once mounted, you'll be able to edit the registry hives to reset your password.
# cd /mnt/Windows/Windows/System32/config
# chntpw -u username SAM
|Resetting Windows password with chntpw|
Replace username with the user account you use to login to Windows. You'll be presented with a wizard-style console interface. It really simple. Just opt to blank your Windows password. The change password option should work well on Windows XP but didn't work on Windows 7 when I tried. When in doubt, just blank the password. It's the most reliable option.
When you're done, just reboot and get back to your usual Windows desktop.
You should now be able to get to your Windows desktop.