Windows XP end of life June 30th 2008
Now Dell in the US have announced that they will be offering Windows XP pre-installed after June 30 2008 via the Vista Business/Ultimate "downgrade" feature, but there are no equivalent announcements being made by Dell UK at this moment.
To avoid any problems, if you are considering buying a new PC in the near future and you wish it to have Windows XP installed, then I would advise you to buy now to avoid any uncertainty.
Please remember that this is the end of "OEM sales" whereby it can be purchased with a PC (the Vista loophole not withstanding). Retail copies are not being pulled off the shelves and the actual operating system will be supported by Microsoft until early April 2014