Are they worth the extra money and where do I find them in the UK? There are several companies on the net in the U.S.A. but I can't seem to locate one over here.
Save your money and buy a Notebook PC. Cheaper, easily available and works very well. I have been using mine on a small 27' Yacht (Albin vega) for the last five years with SEAPRO CHART SOFTWARE (Excellent package). Just make sure it is stapped down with velcro or similar.
Having used a standard desktop PC for over three years as a full-time liveaboard I don't think they are worth the money requested for a "marine". Our installation is in the wheel/dog house hung underneath the chart table with a TFT flat screen 15" monitor that swings down from the roof. Besides chart plotting and running Reeds Electronic Pilot, we have a DVD player, TV card, CD copier, scanner, printer etc.etc., but above all IT IS IN A DRY LOCATION, though at times it has been much colder than in a house.
I use a standard laptop on board as a liveaboard, its kept dry in the wheelhouse and runs all the usual stuff, including charts and weatherfax, e-mail. I've had no trouble with it in 7 years, bought a car charger for it so dont need an inverter, my BJC85 colour canon printer/scanner runs on 12 volts aswell, a very neat package all round. I wouldnt bother with the added expense of "marinisation".
Why not buy or build yourself a mini-pc - this is a cheaper option. You can build a PC using something like the Shuttle SV24 barebones system for half the cost of a notebook of comparable power, and have a system that is easier to upgrade or repair. (The Shuttle is available from www.overclockers.co.uk )
For an off-the-shelf solution an Elenex book PC might be the answer - the ProSentia 1000 can be found on Elonex's site. (www.elonex.com) This machine is VCR-sized and combined with a 15" TFT monitor (available for around £250 and dropping) would make an ideal compact boat system at a very reasonable price.