Restore XP from Vista

Hello,
A friend of mine asked me to help him to get into his locked Sony Vaio laptop PCG-8QEL which has Windows XP. I have a dell laptop with Vista. After researching and finding many solutions that did not pan out, I installed my Vista software to be able to enter the system. I have disabled the password, I believe. Now, I want to remove my copy of Vista and return to the XP. I found the OLd Window files for XP and everything looks in order. There are many different situations I have found on the internet that are close to my problem, but not one exact. I would like to do this properly and hope someone can explain to what would be the best procedure.

FYI: I do not have the installation discs for XP, but I do have the key code.

Thank you, Kimberly