I have noticed many new notebook PCs come with windows xp installed but all desktop PCs have vista. Why is this?

Hi I have windows vista home premium operation system now I want to install windows xp professional operation system. Because some of the software did not work with vista. Now I don’t want to remove vista completely but I want to install xp with vista and worked both operation system.
Is it possible? If yes then How?

My old desktop PC has got windows XP built into it. We recently bought a laptop with windows vista, is there anyway that I can put vista on my desktop, whilst keeping it on my laptop?

My computer is windows vista and every 15 minutes it says not responding when a program like firefox,int.explorer,msn or skype is running. It freezes the whole computer. Vista is so resource hungry and i dont even use that many programs this comp. How can i change to XP?