Cool baby Cool! Then decide...
I felt I had to make a point as everyone, some very technically qualified and some not but being simple home end users, have differing opinions.
Most component parts (ie: CPU processor, hard drives, memory etc) have technical specification sheets and length of expected usage in hours. The main thing to remember is that these very components, all the way down to the memory, last longer if you take extra steps to cool the inside of the tower. You can even buy memory stick coolers that have an aluminum shim clamp and two little fans that blow directly down onto the memory. You can buy hard drive coolers that screw right onto the hard drive and provide cooler temps and longer life expectations for a hard drive, and fit right into the hard drive bays.
Most hard drives are guaranteed for 3 to 5 years and the simple $9.95 added insurance on most sites is well worth the extra $10 bucks as I see it, for there are stories far and wide about "It died a month after the warranty expired". However, the warranty is quite different from what the technical specs give you for "hours of operation" to expect.
A SATA Seagate 750GB hard drive is rated for 600,000 hours of mean time operating hours:
(fourth page - bottom of page)
Mean time between failures
600,000 hours - 25,000 days - 68.45 years
Service life 5 years
Of course we know that hard drives don't operate that long and that is mainly due to shortened life expectancy due to outside environmental influence - ie: no air conditioning on hot summer days - or accidents (surges, manufacturing errors, humidity taking its toll, dropped laptops, etc). The greatest killer of hard drives though is heat.
Ever wonder? Just have some heavy usage going on with your computer (run a movie on your VLC, have several browsers open, be playing your favorite tunes in Media Player or whatever, and let it run for five minutes, and then carefully reach in there and actually put the tip of your index finger on the "non-exposed" side of the hard drive (the side which isn't the circuit face), where it is completely covered by metal (usually the label side). That will answer the question partially as we move on to answer the actual related question of shutting the computer off or on for more improved longevity.
I personally have had a 320 GB Seagate now for one of my several computers I have used for over 10 years and that's 87,660 hours of operating time and I have as a norm left my computers on all the time for just as many years.
As I said, heat is a far worse enemy to your home PC desktop than shutting it off or turning it on from a cold boot, or a laptop, OMG! If you don't seriously consider using a laptop cooler on the bottom underneath of your PLC then you shouldn't even mess with exerting the brain power to read and understand this article.
Most store bought brand name computers have serious inefficient cooling systems. Yet they only have to allow for a few years warranty and those companies make some big ca-chunk change for out of warranty repairs, even some companies charging for any technical help at all for after warranty problems.
Additionally, one cannot forget that even with modern technological changes, and advanced manufacturing techniques, "CAPACITORS" are still yet the hardest hit component of a motherboard affected by HEAT with video chips or bridges being second! Thus many higher end boards have capacitor heat syncs and copper tubing and cooling fins to vent off heat from them. Some boards in the past I have seen (EVGA for one) had small fans integrated right onto the motherboard for cooling the capacitors to some degree.
Remember that the included CPU heat suync and fan often do nothing other than pull heat away from the CPU and board, and end up blowing it right back onto the other components, so adequate cooling fans should be placed in any position available to case cut outs or pepperbox venting areas. You can often buy simple (even LED lit) cooling fans from 80mm to 120mm and larger depending on your case configuration. The important thing is in most "cases" (sorry for the pun) you need to blow as much air in as your suck air out as an exhaust (usually to the back of the case or even out the bottom in some "cases"). You may even find room in the very front of the computer where the hard drives are located, to imp0lement the simplest of mounting systems to attach a fan to cool the hard drive(s).
The on-board integrated chips (South Bridge, video graphics chip etc) can also be modified very simply by purchasing a cooling fan for their specific size and implementation.
Even more important to longevity is buying a good 3.500 to 6,000 Joule surge protector. I strongly recommend purchasing two on sale, then piggy back one to the other. Many think this is silly, for a power line surge or surge from a thunder storm is far stronger and faster than what two protectors would accomplish, but in my own mind's mentality on the subject, in a home situation reality, its not a waste of money.
Buy one, one month, and another later if you find it more to your budget, then give your computer the protection it deserves. You can buy more expensive and higher rated protectors, but in this configuration and for home users, two protectors give you the front (closest to the outlet) surge protector room for all your heavier peripherals (Monitors, sound systems, printers?) that are less susceptible to surge spikes, and the second gives all those other outlets provided for your darling precious computers and such, the most expensive of the system components.
Only as an example:
Better surge protectors work by "redirecting" the surge into a ground, and thus many good ones have an additional eye-washer-grounding wire attached to the plug-in that should be installed into the center screw hole of your outlet face-plate on the wall. Surges last from a mere millisecond to a mite nano-second. Thus using two as a piggy back setup allows the added protection as the surge hits, redirecting power and any surge getting through in that fraction of a second is redirected by the additional staged protector.
When it comes to longevity of your computer by running it all the time or shutting it down, its a toss up for you if you wish to know the truth. If you work an 8hr day, and live in a region where electricity is $.11 - $.14 per Kilowatt hour, then you are looking at a whole $1.96 for a full days electricity in most situations, enough to run your computer for a week most likely since your power supply on a standard Dell or HP or eMachine or Compaq is only from 350W to 500W. That's "IF" and we mean "IF" your computer is running wide open using every bit of energy it can suck, all day long.
I figure I use about $.75 a day on mine and to me its worth it. Put your computer on standby, hibernate, or sleep mode and you use even less power and preserve many of the internal component's integrity.
The main thing is, COOL THAT COMPUTER! Shop around and find the sites you like for case fans and add-on mods to help keep that baby cool!
Then think VOLTAGE and buy some good surge protectors!
If you are home a lot, protect those capacitors and avoid turning the computer on or off all the time, especially in colder climates or constantly air conditioned environments.
Remember that most chips and computer components operate between 30*C and 60* C. That's from 80+*F to a whopping 140+*F. A cold boot from normal environmentally controlled room temps is a drastic change when comparing health of computer components. Someone said that swelling of computer components and fracturing or line cracks in system buses is not an issue. Believe me when I say many a computer has failed from a swollen or overheated capacitor, or a video or bridge chip losing its structural integrity and weld due to inadequate cooling and overheating. Just Google "laptop repair" and see how many ads are out there from computer repair shops, for resetting copper chip heat transfer plates and re-soldering chips onto the boards.
So use common sense, use the software tools, programs and utilities included in most operating systems to put your computer into energy saving and parts wise sleep modes and enjoy a longer life, less expensive computer experience.