First, make sure that your computer is completely virus/malware-free:
Once you know your machine is clean, you can log into the built-in
Administrator account and make any user account changes you need.
In XP Home, boot the computer into Safe Mode. Do this by repeatedly
tapping the F8 key as the computer is starting up. This will get you to
the right menu. Navigate using your Up arrow key; the mouse will not
work here. Once in Safe Mode, you will see the normally hidden
Administrator account. The default password is a blank.
In XP Pro, you do not need to go into Safe Mode. At the Welcome Screen,
do Ctrl-Alt-Del twice to get the classic Windows logon box. Type in
"Administrator" and whatever password you assigned when you set up
If you reset the built-in Administrator account's password in Home or
have Pro and don't remember the password, use NTpasswd to change the
built-in Administrator account's password to a blank. Then go to the
User Accounts applet in Control Panel and set passwords that you will
remember and make other desired changes.
Elephant Boy Computers
MS-MVP Windows - Shell/User