If you want access to your Windows account (and not just access to your files), you can reset your password quite easily. Learn, in this guide, how to burn a password reset disc and how to boot from it and reset your Windows account password.
Warning: Do not reset your password if you’ve used Windows to encrypt your files. Instead, attempt to crack your Windows password to regain access to these files.
Burn the Windows Password Reset Disc
The first step is to download and burn the Offline NT Password & Registry Editor
1. Download the disc here (scroll down to “Download” and download the latest CD release)*
2. Extract the zip file and burn the ISO file to a disc (how to burn an ISO to disc)
*You can use the USB version if you would prefer
Reset Your Windows Password
1. Put your password reset CD in the PC that has the locked account and boot the PC from the disc (how to boot from CD)
You will now see a series of prompts. Follow the instructions (the default choices are in [brackets])
2. Type the partition number where Windows is installed from the list of Candidate Windows partitions found (usually 1 or 2) and press Enter
3. Press Enter to use the default Registry directory (unless you changed it, this will be right)
4. Type 1 to select Password reset and press Enter
5. Press Enter to Edit user data and passwords 
6. Type the username of the account for which you want to change the password. i.e. Rich and press Enter
7. I recommend you clear the password. To do this, type 1 and press Enter
8. You will receive confirmation that your password is cleared. Now type the same username again and press Enter
9. Type q to quit and press Enter
10. Save the files by typing y and pressing Enter
11. Press Enter to quit
12. Now restart your machine
13. Wait for your machine to boot and login without a password
Be sure to set a new password and create a password reset disk to prevent future loss.
Last Updated: June 19th 2012
2012/06/19 — Updated screenshots to match latest version. Added warning about encrypted files.