Handheld computers have been around for a while now, but OQO makes the most unusual ones we’ve seen.

Most handhelds run a special, simplified operating system that includes just a few functions such as email and a contacts list.

Despite measuring just 142x84x26mm, the e2 runs the full version of either Windows XP or Windows Vista. It's a retooled version of OQO's original PC, which we reviewed back in 2005.

Running Windows is possible because, despite its tiny size, the e2 contains most of the things you would find in a desktop or notebook computer. Several models are available, but the version we looked at has a 1.5GHz processor, 1GB of memory and a 60GB hard disk.

When closed the e2 will fit into a large pocket, but to use it you slide up the screen revealing a Qwerty keypad that’s designed to be operated with your thumbs, in much the same way as sending a text message on a mobile phone.

A notebook-style touchpoint makes it easy to operate the mouse, but we found that the wide keypad took some getting used to and, even then, it wasn’t particularly comfortable. The keys are at least quite large, and the Control, Shift and Alt keys have been designed so that they’re easy to use.

Even though the e2’s keypad is disappointing its screen is great. It has a resolution of 800x480 pixels, and a glossy coating that makes colours look vibrant (although this does make it susceptible to fingerprint marks and smudges). The brightness is easy to adjust using a special utility that's supplied with the computer.

Wireless networking and Bluetooth are built in, and we found it easy to connect to the internet and surf the web using the Firefox browser, which is also supplied. It’s great to have access to all the programs you would use on your main computer when on the move, and we found it was easy to take notes using Word then copy them to a USB key using the socket on the bottom. There's no CD drive, which means you'll need to install programs using a USB key or over the internet.

The e2 is certainly an impressive gadget, and if you need to run Windows applications on a computer that fits in your pocket it’s the only real choice at the moment. On the other hand it gets annoyingly hot, and the battery runs for only a few hours. For most people, a small notebook is a better bet.

Vista compatible: Yes