Try following trick which is actually a loophole in Windows XP Setup and a big security hole:
A. Boot using Windows XP Setup CD and follow the instruction like Accepting EULA, etc.
B. When it asks to repair your existing Windows installation, accept it and press "R" to run the repair.
C. Setup will start repairing your Windows and will start copying files, etc.
D. After a few minutes setup will restart your system and when it restarts don't press any key when it shows "Press any key to continue..." otherwise Setup will start from the beginning. Don't press any key and setup will resume where it left.
E. Now it'll start doing other tasks and will show a small progressbar with a few details in left side.
F. Look carefully at the details and when it shows "Installing devices", press
G. It'll open a Command Prompt window. Now type nusrmgr.cpl and press
H. It'll open the same "User Accounts" window which you see in Control Panel.
I. Now you can remove or reset any account password without any problem.
There are many 3rd party utilities which claim to recover forgotten Windows password:
Windows Password Recovery Tool Service:
Windows Password Recovery Tool is a program that works instantly to remove any lost or forgotten password. Simply boot your PC from the supplied Boot CD, and get access to forgotten Windows and administrator passwords in just seconds. Their method is so much faster/easier, and works great!