I am going to show you some ways you may get admin access to the XP or change password of another administrator. In fact, none of these work in 100% condition but they are worth giving the try and we may have luck sometimes.
1) When you boot the computer and come to the login screen, press “Ctrl+Alt+Delete” and now the box where you enter your login username and password thru keyboard comes instead of that icon to click on. Now type “administrator” in the username field and leave the password box blank. Press Enter. And you might get logged in. It works in many cases as I have seen that many forget[or don’t care] to put password for administrator account while installing XP. Give it a try and you may own your friends.

2) Suppose you have administrator power and another friend or your brother has got account of administrator right on same computer. Now you want to change the password of his but you don’t know the password. Don’t worry. Login to your account and start command prompt. Now type following in the cmd.
net user Username_whose_pass_you_wanna_change *
Eg: net user administrator * //to change the password of user called administrator
then press enter

3) How to add new user/password from command prompt:
net user /ADD This adds new user with specified password
eg: net user sam207 mypass /ADD This adds new user sam207 with password mypass

net localgroup administrator sam207 /ADD This adds sam207 to the administrator group.

4) The C:\windows\repair\ contains backup of password file sam and you may grab it and crack it with l0phtcrack or some other crackers available on the internet.