Use XP Pro SP3. Your machine is too slow(I assume the 1.25g is referring to RAM, not hard-drive) to handle Vista, and XP-home is not supported by Graphisoft. Also, ArchiCAD 12 isn't going to run very well on your hardware anyway. that machine must be at least 4 years old right?
Of course, you can't buy XP anymore though. What OS do you have at the moment?
Josh Osborne - Central Innovation
HP Zbook Studio G4 - Windows 10 Pro, Intel i7 7820HQ, 32Gb RAM, Quadro M1200