Windows XP PRO doesn't accept original OEM key

LaDidi or Experts in here,

Why is there a need to break them into 3 parts since the sum of each hex-byte of the string will produce the last byte as checksum??

Where/which articles did you manage to lookup on this 'special' info from?? There're lots regarding PID only..

I did a check for Win2k3 and it's true that by summing each hex-byte together will produce the last byte as a checksum..

Btw, the main question: What's/Where's this ExtraData use for??

I found that it's different for different type of the same release such as SC, TC and Eng are diff in value..

I was wondering if that string has any relation with the product key/id.