You can use safe mode to get to Windows' built-in Administrator account. By default, the Administrator account does not require a password. You can use this built-in account to snoop through other user's files and give administrator privileges to any account. To start the computer in safe mode press F8 as the computer is starting. If that doesn't work, try pressing the Ctrl key during bootup.

This is a serious security flaw in Windows XP. The welcome screen does not indicate whether accounts have administrator or limited privileges so anyone can make changes to their account without anyone knowing. You need to protect this built-in account with a password.

To do this, log into the built-in Administrator account. Click Start>>Control Panel. Double-click User Accounts. Select the Administrator account and choose "Create a password." After you set the password, you'll also have the option to set a brief reminder message in case you ever forget the password.

While you're there, make sure that other administrator accounts on the computer are protected by passwords. Including your own account.