Trying to install Premiere Pro 1.5 and Upgrade to 6.0 but hitting Authentication issue

I am installing Premiere Pro 1.5 on an old Win XP pc and I have the 6.0 Upgrade disk BUT I cannot authenticate Pro 1.5 online. I see the comment about the CS2 upgrade and I have downloaded the free Premiere Pro 2.0 version but it does not allow me to upgrade to my Pro 6.0. Is there a work around to this? I would like to use the newer software to take advantage of the improved features.