oem vs full .. oem you get a disc.. full you get a chessey book and a 1-800 number to call for no help. need i say more.. and yes you can upgrade an oem version of vista that was preinstalled...
Now i have installed an oem on another machine. but the i had to take the old one off the internet to activate the new one. It was with the first ed. of XP Media Center Ed. 2002