actually oem means original equipment manufacturer. where u only able to install windows xp on a new formated pc without a os inside it or a os inside it but i will now able to upgrade the os to xp. u have to run 2 os on that pc. example windows me and xp.
the for windows xp there are 2 version , one is home edition and another is pro.
for both of the os there are 2 type of release , that is oem, retail. with retail you will able to upgrade or clean install the os.
are you clear now ?