Scenario - you just bought a laptop on eBay and it is protected with an administrator password... you don't know the password so the laptop is useless; right? Not so fast!

Depending on the operating system you are using, it can be quite easy to solve this problem.

Windows XP (EASY)
1. Boot into Safe Mode - press <F5> before the Windows screen appears
2. Select SAFE MODE
3. You will see a user you have not seen before (Administrator) that does not have a password
4. Login with Administrator
5. Change password of the user account (or create a new one)

Windows Vista / Windows 7 (HARD)
1. BE CAREFUL... this can get complicated and can wipe out your system files if not done properly
2. Download Ubuntu Linux from another computer
3. Burn the .iso to CD
4. Boot from the CD (you will have to change your settings in the BIOS to boot from CD FIRST)
5. Choose TRY UBUNTU
6. Install Software
7. Select System > Administrator > Software Sources
8. Click the two boxes not checked (universe and multiverse)
9. Close and Reload
10. Select System > Administration > Synapotic Package Manager from menu
11. in the window's search field type chntpw
12. A program will appear... click on it
13. Select Mark for Installation
14. Click Apply
15. Click Apply
16. Mount Drive
17. From menu choose Places (your internal hard drive or Windows partition)
18. A File Browser will appear (like Windows Explorer)
19. Double-click on folder icons
20. go to <Windows drive>\Windows\System32\config folder
21. Leave this window OPEN
22. Choose Applications > Accessories > Terminal from the menu
23. Arrange the two windows side-by-side
24. in Terminal type cd /media (yes there is a space after cd) then the path to your config folder (back in Step 20)
25. Press Enter
26. type sudo chntpw -u logon SAM logon is the name for the administrator account
27. Press Enter
28. Press 1
29. Press Enter
30. At the question type Y

When the program is done, reboot into Windows... the account no longer has a password on it

Please note for Vista/7, if you feel uncomfortable at all doing this, it is best to leave it up to a professional to unlock the administrator password. If you have XP, you are in luck... it is quite easy.

Source: PC World Magazine (2/11)