if you already have a machine with a certificate of authenticity sticker for windows xp, you can use any xp installer around (provided its the same version as that which was originally installed in the machine, like Home, MCE, Professional), and then use the Microsoft Product Key update tool from microsoft to change the key to the one on the sticker.
perfectly free, legal windows installation. oem nga lang yung key (VLK), but the good part is that its pre-activated na. no need to call microsoft. the machine is already pre-installed with XP kasi, so part of the cost of the machine went to the license.