If you have XP Pro on your computer, at the log in screen type
ctrl-alt-delete
twice.  In the user account type administrator, leave the password blank and
press enter. You likely didn't create a password for this hidden account
during setup and if the computer was purchased with XP pre-installed it
doesn't likely have one either,

If you have Home, boot into Safe Mode (F8 after the post test and before
Windows starts.  Once Windows starts loading, you've missed it and will have
to try again).  You'll have an option to log on as Administrator.  Again,
the same procedure, you'll be asked to log in to get into safe mode, type
administrator, leave the password blank and press enter.

This will get you in.  Then go to Control Panel, user accounts, select the
specific user and select change password.  Once you've changed the password
it would be advisable to create a Password Reset Disk by clicking "Prevent a
Forgotten Password" link in the left pane.