Vista looks nice and pretty but is not that much difference that XP with Service Pack 2 installed - Vista just has afew more bells and whistles in it.
I can think of no valid reason why at work I would want to upgrade the 1500+ computers that we have. We shall stick with XP SP2 for the time being.
One thing I would say though, never do an upgrade to Vista - always perform a fresh install as its a hell of a lot quicker! 40 minutes to do a clean install or 3+ hours for the upgrade... go figure!
