Panel PCs are available as touch devices with 10.4-inch VGA, 12.1-inch SVGA and 15-inch XGA TFT display. Housing can vary from very flat devices without PCI slots to expandable devices with two PCI slots. Additionally, the Panel PC has an integrated Compact Flash slot, which can be expanded with a second Compact Flash slot or hard disk. The Panel PC was designed for operation without a fan. Windows XP embedded is the modular variant of Windows XP professional, and is the preferred Panel PC operating system with low memory requirements.