Bought a used pc, do not have password for windows

The first thing which you check if you forget login password. When we install Windows, it automatically creates an account "Administrator" and sets its password to blank. So if you have forget Your user account password then try this:
Start system and when you See Windows Welcome screen / Login screen, press ctrl+alt+del keys Twice and it'll show Classic Login box. Now type "Administrator" (without quotes) in Username and leave Password field blank. Now press Enter and you should be able to log in Windows.
Now you can reset your account password from "Control Panel -> User Accounts".
Same thing can be done using Safe Mode. In Safe Mode Windows will show this in-built Administrator account in Login screen.
Windows XP and further versions also provide another method to recover forgotten Password by using "Recovery Disk". If you created a Password Recovery Disk in Past, you can use that disk to reset the password. To know more about It, please visit http://www.windowspasswordsrecovery.com

if you have try all the free way and they could not help.
you should try http://www.resetwindowspassword.com/
the cheapest recovery tool.
and I succeed to log in.

Most XP machines have a built in administrator account that normally does not have a password.

You can try pressing F8 before Windows starts up to go into the XP bootup menu.

Choose Safe mode

Once into safe mode you should either see an 'Administrator' account listed, or just type in Administrator and do not type a password. If that works, go to Start-->Control Panel-->User Accounts. From there you can create a new user account for you to use with administrative privileges.

If that doesn't work, try downloading a password reset utility, such as the one at http://home.eunet.no/~pnordahl/ntpasswd/

Hope this helps