Getting to XP passwords..
If you do a bit of Googling, you will likely come across a Linux based tool-set that allows modification or clearing of any password on a local basis - ie - you have to be able to boot into the machine directly.
What it does is boot the Linux kernal, then it knows where the password files lurk, and it knows how to edit them, neatly bypassing the Windows protection and user-abstraction layer. I have it on a CD, which will boot off about any machine with a CD drive, and have used it many times to access machines that we don't have an administrator password for. Once the passwords are cleared - you can boot the machine into the operating system (XP, W2000, W2003, Vista) and go make whatever changes you need to make - including a new administrator password.
Here is a spot to start: http://home.eunet.no/~pnordahl/ntpasswd/