See what is applicable to your situation.
XP Pro on your computer:-
1. At the log in screen type " ctrl-alt-delete " twice.
2. In the User Account, type Administrator
3. Leave the password blank and press enter.
(Most likely a password wasn't created in this hidden account during setup).
4. Go to Start - Control Panel - User Accounts.
5. Select your name and click change the password.
If you decide to set a password it is recommended that you create a
Password Reset Disk by clicking on "Prevent a Forgotten Password" link in
the left pane. The Wizard will guide you through.
http://support.microsoft.com/default.aspx?scid=kb;EN-US;321305
How to Log On to Windows XP If You Forget Your Password or Your Password
Expires
In Windows XP Pro - Password Expiration
Click Start, select Run
Then type "LUSRMGR.MSC" without the quotes
Go to the Users folder and double click your entry.
Set your user to Password never expires.
Password Has Expired Message
Start/Programs/Administrative Tools/Local Security Policy/Account
Policies/Password Policy. In the right pane, right click, properties,
modify (use accordingly). And Start/Programs/Administrative Tools/Computer
Management/Local Users and Groups/Right Click
"User"(intended)/Properties...Or with Admin privileges, at a command prompt
type: net accounts /maxpwage:unlimited.