My son has a friend that either forgot his XP login password or someone changed it and he didn't know what to do to access his computer. What he has to do is login in using the Administrator account .
The Administrator account is not displayed in User Accounts unless the computer is in Safe mode. To change the password for the Administrator account:
1. Click Start, click Turn Off Computer, and then click Restart.
2. After the computer completes the Power On Self Test (POST), press F8, and then click Safe mode.
NOTE: If you have difficulty getting to the Safe mode menu command, press F8 repeatedly after you turn on the computer.
3. Log on as an Administrator, or with another account that has Administrator permissions.
4. Click Start, click Settings, click Control Panel, and then click User Accounts.
5. Click the Administrator icon.
6. Click Create a Password or Change my password.
7. Type a password for the account, and then retype the password to confirm it.
8. Type a hint to help you remember the password in the event that you forget it.
9. Click Create Password or Change Password.
For additional information about how to create and configure user accounts in Windows XP, click the article number below to view the article in the Microsoft Knowledge Base:
279783 HOW TO: Create and Configure User Accounts in Windows XP
There is also a great post over at lifehacker.com on how to change your password from the command prompt. Go take a look at it.