|worldlibrary has made sound suggestions but not necessarily in the correct order.|
Most laptops come with a hidden recovery partition and a mechanism to restore the Laptop to factory default status. If this is the case then this is the best route for what you want to do. This method allows you to give your son a factory fresh laptop just as it was when you purchased it.
If there is no recovery partition or it has been removed then you can install XP cleanly using an XP install CD. Again if it is an OEM laptop it is unlikely that they will have given you an XP install CD. They may (if you are lucky) given you an OEM re-installation CD which is good enough and can be used to re-install XP.
If neither of the above options is possible then you will have to do the "clean" manually. This will be firstly uninstalling any programs (using add/remove programs) that you do not wish to leave on. Next you will have to delete all your personal data which falls into 2 broad categories:
1. Media files/pictures/videos (essentially stand alone files and directories). With these you can do a straight forward delete of either the files or directories. Bear in mind that files/directories deleted in this way are recoverable even after you have emptied the recycle bin. You will have to use file scrubbers if you want to prevent that possibility.
2. Data associated with programs such as email clients. For these you have to first go into your applications (outlook in this example) and delete your files. Once the files are all deleted then delete/remove the email account itself.
It may also be a good idea to clear your history, cookies and cache in your web browser.
☺ When everything else fails, read the instructions.