Best O.S. for Archicad Is : WINDOWS 2003 SERVER ENTERPRISE
tweaked with settings not for servers but for workstations.
Greatest speed compared to XP an Vista ....without bells and whistles
but fast and responsive like a race car . It,s the best and mature operating system even you have a slow computer. It give you the chance to ad to your system more then 4 gb of ram (how much your motherboard support ) and is a 32 bit os not like xp 64 .
At the address :
http://www.msfn.org/win2k3/
you will find a great guide how to convert your Windows Server 2003...
to a Workstation Os for every desktop pc !