Manufacturers have been putting desktop processors inside notebooks for years, so in a sense the presence of a 2.4GHz desktop Pentium 4 inside IBM's new Thinkpad G40 is hardly ground-breaking. What is interesting is the way in which the reasons behind the choice of a desktop CPU for a notebook have changed.

Back in the day, the usual motive for using desktop silicon in a mobile was speed. Mobile-specific CPUs were invariably slow because high clock speeds generated more heat and sucked up battery power, but this didn't deter some people from using a desktop processor then claiming to have the fastest notebook on the market. That it ran for about 25 minutes on DC power and could easily bake your privates if you had it on your lap was conveniently overlooked in the excitement over those extra megahertz.

Of course now we live in more enlightened times, and we have extremely fast notebook-specific processors like the Pentium-M and its predecessors, the Pentium III-M and 4-M. We don't need desktop chips for speed - so why did IBM stick one inside the G40?

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The simple answer is money. Pentium-M series chips are great, but they cost, and that premium is reflected in the price of the notebook. IBM has taken stock of the market and decided that what it needs is a decent but affordable portable which can act as a desktop replacement. How to do this? Simple: stick in a desktop CPU - bags of power for way less money than Pentium-M.

This had us eyeing the G40 with some scepticism. After all, we remembered the days of joke battery life and singed boxers when desktop CPUs took the stage, and we were a bit dubious. We need not have worried. The G40 really is a desktop replacement in the sense that it's too big and hefty to carry around much. It measures roughly 33cm x 28cm (W x H), and it's a real tombstone - 5cm thick at the back tapering down to 4cm at the front. The scales tipped at a shoulder-popping 3.9kg which rises to 4.5kg when you add in the breeze block of a power supply.

The size and bulk are not gratuitous; high-speed CPUs need efficient cooling, and IBM has made sure that there's plenty of room inside for big heatsinks and lots of airflow. There's a whopping great fan in there as well to keep the air moving, but on the odd occasions when it activated it ran very quietly. Big notebooks might not be much use to the dedicated traveller, but ergonomically they tend to run rings round smaller machines. This is certainly the case here, what with the 15-inch TFT screen and the large, almost desktop-sized keyboard.

The screen runs at 1024 x 768 resolution, which is very readable and practically sized on the 15-inch diagonal, and the spacious keypad incorporates large keys where you would expect them and is very easy to adapt to. The keyboard is also angled down towards the user because of the notebook's wedge-shaped profile, which greatly improves the typing angle and reduces strain on the wrists and forearms. In short, this is a notebook you can do real work on, which is a vital consideration if it is to stand up as a true desktop replacement.

There's plenty of connectivity on offer, although the Thinkpad lacks a dedicated interface for a docking station, which is a shame given how useful they are, but not the end of the world. Still, you get a generous four USB ports, a parallel port and even a PS/2 port for a mouse, now something of a rarity.

You can add one PC Card to the system, and the double-height slot means that you can use a Type III removable hard disk card as well as normal Type II cards. Communicating with the rest of the world is accomplished via the integrated V.92 modem, which sits alongside a 10/100Mbps LAN adapter.

There's no infra-red, no TV output and 802.11b wireless networking is an option on some models, though not this one. Again, none of this is likely to scupper the G40 unless you have very specific requirements. The floppy and 8-speed DVD-ROM drives are built in, so you won't be able to swap them out for fancier alternatives as you can with some notebooks, but an external CD-RW, for example, could be hooked up easily enough if needed.

The core specification is about what you'd expect for the price: 256MB of PC2100 DDR memory backing up the 2.4GHz Pentium 4, a 20GB, ATA-100, 4,200rpm Hitachi hard disk, and graphics courtesy of the Intel 855 motherboard chipset. The graphics subsystem borrows memory but at least it only grabs as much as it needs, on a sliding scale between 8MB and 64MB. We weren't expecting too much from the chipset in 3D mode, and although it works, it certainly isn't the avid gamer's dream. You'll be fine if you stick to 2D and business applications, which is after all what this notebook is meant for.

Speed-wise, you are looking at a notebook that's fast enough for normal everyday work rather than a new standard by which others are judged. We'd hoped to see a bit more fire and smoke from the processor (not literally), but we still felt that the machine is more than up to the job for which it was intended.

This is the crux of it really, this matter of being fit for the job. The G40 may not be especially glamorous, and it doesn't have Centrino technology or any unnecessary frills, but this doesn't mean it's not a good product. On the contrary, it seems to have been painstakingly designed to work with a desktop processor inside, with cooling and even battery life issues carefully resolved. In fact we ran it on DC for over 3.5 hours non-stop, which is really very good going for something this power-hungry.

So long as the G40 is what you want - a quality, branded desktop replacement for work rather than play - then you will probably be very happy with it. Especially if you thought you were going to have to spend £1,500-plus on a notebook.

The IBM Thinkpad G40 is a really rather good desktop replacement with the ergonomics you need for doing proper work rather than just pecking away for an hour or so. A notebook-specific processor has been sacrificed to keep the price down, but in our view this is a price well worth paying.