For the first time in two years Sony is updating its Vaio U ultra-compact PC line with a model that is the smallest Vaio yet developed, the company said today.

The UX50 packs a Windows XP-based PC into a case about the size of a paperback book. Like previous models it has a touchscreen display. This model's screen slides up to reveal a full qwerty keyboard underneath. The keyboard was added because some customers expressed frustration with the previous model that using the touchscreen was troublesome, said Yoshihisa Ishida, head of Sony's Vaio division, at a Tokyo news conference.

The model announced today has a 30GB hard-disk drive, but Sony is also developing a model that uses 16GB of flash memory in place of the disk drive. More details on this computer, a prototype of which Ishida showed today, are due in late June and the computer will likely go on sale in the middle of this year, he said.

Compared to a hard-disk drive, flash memory uses less power, is lighter and smaller however drives are more expensive. That coupled with supply constraints is one reason why flash memory hasn't been put to more widespread use in computers, said Ishida.

The flash-based Vaio U will be more expensive than the hard-disk drive-based model, he said, however he wouldn't disclose any pricing details of the yet-to-be announced machine.

Currently a 16Gb flash-memory chip is selling for about $36 (£19) on the spot market, according to prices quoted by DRAMExchange. That would make the cost of the chips alone for the 16GB drive around $288 (£152) if they were purchased on the spot market.

Other features of the UX50 computer announced today include an Intel Centrino Core Solo U1300 (1.06GHz) processor, 512MB of memory, a 30GB hard-disk drive, wireless LAN, Bluetooth, a built-in camera, fingerprint sensor and reader for Felica contactless smart cards. It measures 150x32x95mm and weighs 520g.

It will go on sale in Japan on 27 May and in the US in July. In Japan it will cost around ¥170,000 (£820) and in the US will cost about $1,800 (£950), Sony said.