PARANOIA originally relaunched as XP, then it became XP Service Pack One… and then the XP vanished, because it wasn’t worth making Microsoft grumpy. For collectors, the middle copy is the holy grail, because so few of that edition exists (if any!), and purists might prefer to cling to the original XP edition (because how could The Computer have been wrong?!).
Anyway… you can – apparently – still get the XP edition off of DriveThruRPG as a .pdf. Same price as the Service Pack One edition (without the XP). If you feel that way inclined. Preview pages certainly hold up to scrutiny (i.e. this doesn’t appear to be an administrative error). So, by all means – avail yourself of this original version to complete your collection.