As we've previously mentioned, as of June 30 Microsoft is no longer be selling boxed copies of Windows XP or allowing manufacturers to install it directly (although some white-box vendors are immune). If you want to stick with XP, you'll need to purchase a machine running Vista Business and then get a downgrade licence to revert to XP.
Fran Foo at AustralianIT rang around local vendors to find out how much such a downgrade would cost. Pricing ranged from nothing (for HP models) to $83 to get an XP CD from Lenovo. If you decide to stick with Vista instead, check out our Vista hub for tips on how to make the most of Vista.
Local vendors gear for Vista downgrade [AustralianIT]