Two things.
Firstly, VERY IMPORTANT, make sure you dont delete the adminsistrator user (not sure if XP lets you).
A user can either be an administrator or a limited user. If you delete the adminsitrator but keep only a limited user then you wont be able to install any programs, and you cant make your self an administrator if you are a limited user so you may have to reintall Windows.
Also, if you delete a user all their personal files will be deleted also. Make sure you you rescue any personal files from My Documents before deleting the user.