Microsoft has started shipping its first Surface tablet computers ahead of their official launch on Friday. The firm's UK site says it has sold out of pre-orders for the 32GB version of the device, which runs the RT version of Windows 8. It marks Microsoft's entry into the PC hardware market, posing competition to other computer manufacturers. Early reviews of the device have been mixed, praising the hardware but criticising a lack of software. Surface with Windows RT - to give the device its full name - runs on a chip made by Nvidia, based on the designs of British company Arm Holdings. It is designed to be cheaper and offer longer battery life than products using x86 chips that run the full Windows 8 system. The caveat is that Windows RT devices can only install third-party software from Microsoft's own Windows Store. Although they do offer a traditional desktop mode, the only programs that can be run under it are Microsoft's Office 2013 suite and a limited number of the firm's other products. Until more third-party apps are developed for the touch-interface mode, users face a significantly smaller selection of software than they would on traditional Windows devices and tablets powered by Apple's iOS or Google's Android operating systems. Microsoft intends to launch an Intel-based version of the Surface offering the full Windows 8 system in January. But it does appear there is demand for the Windows RT. Although Microsoft has not disclosed sales figures, it said that pre-orders of the 32GB version of the tablet - which costs £399 - had "temporarily sold out" in the US, Canada, France, Germany and the UK within a day of going on sale. A £559 64GB version bundled with a Touch Cover keyboard remains on offer.