User Accounts option in Control Panel section of your Windows allows assigning password protected to the respective account you desire to. Meanwhile, here are a few set of tips to reset administrative password of your system in case of unavailability.

Reset Administrator Password Manually for Windows XP

NOTE: Although, the process is for a Windows XP version of system but it is recommended not to use boot CD of the respective version of Windows. This is because Recovery Console will demand for a password to process offline installation. However, boot CD of any other Windows version can be used for instance; Windows 7 to reset an unavailable administrator password on XP version of Windows.

This works as the Windows Recovery Environment for Windows 7 or even Vista allows launching command prompt along with the access to offline installation provided without password being demanded for the same.

Follow these steps for resetting password of an Administrator account on your system.

Boot from Windows RE and run command prompt window.
Find the respective letter assigned to the drive partition which holds installed Windows. Type C: and start searching for respective Windows folder.
Now type in the mentioned command and replace the letter C: with the respective drive letter if it isn't C: in your case. Command: copy c:\windows\system32\sethc.exe c:\
This will create a replica of sethc.exe file available in your Windows' System 32 folder.
Now type in the following command to switch sethc.exe with the cmd.exe. Command: copy /y c:\windows\system32\cmd.exe c:\windows\system32\sethc.exe
Then reboot your system and once you get to the logon screen; keep pressing SHIFT key up to five times.
You will now see a window for command prompt in which you have to enter the mentioned command to reset password for Windows. Command: net user your_user_name new_password (Mention the user name in place of "User_name" and a new password in place of "new_password")
In case the user name is not known simply type in net user for listing available user names.
Now log into the account with newly generated password.