Dell pre-installs two hidden partitions on the harddisk; the idea behind them is sound as it eliminates the need to remember your XP’s product code and automate the activation process and when something goes really wrong, the option to restore the computer to its original factory state is always there. Re-installation of the OS is done simply by expanding a backup image file. There is no need to look for any device drivers and all the pre-installed software are there the first time you boot back to Windows after re-installing it. But in order to automate the process and to allow the user to access the hidden utility partition, Dell uses its own partition structure which unfortunately is not what a typical normal OS will see. And it includes Windows XP – the OS Dell has pre-installed for you.
http://www.hollymolly.net/ca/2006/11/dell_ubuntu_dis.php