Okay. This is a simple question, with a simple answer. But I'm second guessing myself: Let's say I have 2 computers running XP, but I only have one copy of XP on CD. That's b/c my second computer came with XP on it but didn't have a CD for it. If I wanted to format my second computer, could I use the disc I used on my other computer PROVIDED I use the serial # on the XP tag on my second computer? I've heard different answers from equally reputable people...does anyone know for sure? Does each copy of XP have it's own electronic serial number burned into the CD?