I have had computers on my boat for 10 years, onto my third desktop now, None have ever failed. The only reason was to replace was to keep up the speed and processor requirements. I must say the equipment is in the lounge (oops, saloon), and run from either shorepower or inverter.
I have now a Shuttle system with widescreen LCD. Still a lot cheaper than laptops, with similar specs.
I agree for navigation one does not need to have the latest and fastest, but most of us have now digital camera and videos, we are on the internet (wireless), we download music, and we use the computer for entertainment. I have a TV card (alhtough seldom watch TV). Therefore system requirements are simliar as to a normal 'home/business' environment.
If one wants a screen outside or at the helm, why no buy a second one?
I guess to bring a laptop or any computer outside is not advisable.
I like the idea some of the previous posts to invest in 2 or more identical computers, but that is driven by the expectation that they will fail, rather sooner than later.
The best prevention would be to keep the moisture out!!! In the past I had only one problem: the computer would not boot on very cold mornings (with a lot of dew outside). If I left my computer switched on on the previous night, then there was never a problem! I agree with Valiente.
Leaving the computer on, may be only problematic with a limited supply of current, when under way. I read somewhere that cruisers spend 90% of their time in harbour where supply of electricity is not a problem.
Why would one invest in mulitple computers, or in high priced mil spec laptops when a normal desktop would do? It only means that the desktop takes up more space, and uses a bit more electrical juice whilst sailing.