General rule of thumb for all PCs: The colder it is, the better it runs. Some jokingly say that a computer runs best with a "frosty dew" on it. The reason that’s a joke is because condensation in any form that’s physically on the motherboard (aside from cooler units) is obviously bad, because water and electricity don’t mix.

General rule of thumb for computer monitors (all types): Operates best at room temperature (72 F/22.2 C) while not in direct sunlight.

General rule of thumb for laptops (all types): Typically you will find at least one "spot" on a laptop that’s warmer than the rest of the unit after it’s been running for a while. It differs in location dependent on model, and the spot that gets hottest is usually where the processor is. Your best defense against laptop overheating is to ensure the fan is clean. Spraying dust removal spray lightly on the fan while the laptop is OFF (obviously) is usually the only way to clean it. If the vent slots are thick enough you can also use a cotton swab (laptop has to be off for that also).

How to deal with extreme temperature situations

Cold (computer): If any computer is in a very cold environment and has developed a frost on it, wipe what you can off the case, DO NOT turn the unit on and let it sit for a good 20 or 30 minutes to let the case adjust to room temperature before powering it on.

Cold (laptop): If a laptop is cold enough, the keyboard may start to curl (literally) at the corners and the touchpad won’t work at all because the sensor just won’t operate at that temperature. You need to let the unit warm up to room temperature first in the off state before powering up, else you risk damaging components. In addition you may notice it’s difficult to open due to the cold "flexing" the hinges. If when you start to open the laptop lid you hear cracking/rubbing noises, STOP. Close the lid and wait for the hinges to "flex back" before opening again.

Cold (CRT monitor): Unless there’s frost on it, a CRT can usually be powered up even in the coldest of temperatures. The screen will show a very dim picture until the tube warms up.

Cold (LCD monitor): LCD monitors are usually very forgiving when it comes to cold. However with frost on it you should let it adjust to room temperature first before powering it on to avoid condensation damage. You will also notice a dim picture on startup because the backlight bulbs haven’t quite warmed up just yet.

Heat (computer): In an extreme heat situation you can open the case to "air it out" first for about 10 minutes, then close the case up and start the computer. IMPORTANT NOTE: Running a case open does not cool it better because the air flow from the fans is wrecked at that point. You need that case closed so the air gets to where it needs to be. If the computer still runs too hot, you need better fans and/or a case that has better air flow and/or the addition of a cooling system.

Heat (laptop): Same situation as a desktop PC. Open the lid, let it sit and adjust to room temperature first before turning it on. You will know it’s ready to turn on if you touch the LCD screen and it doesn’t feel hot to the hand. Otherwise, wait until it cools off. It will usually cool off quickly.

Heat (CRT monitor): Ordinarily there is no danger in starting a CRT monitor even if it’s been "cooked" a bit from extreme heat. However if the enclosure holding the tube feels hot, you should wait until it cools down first before turning it on.

Heat (LCD monitor): LCD screens will run even in the most extreme heat because they don’t produce that much heat to begin with. What to watch out for is the warping of the screen enclosure. But this is rare and basically never happens unless the environment is so hot that is starts to warp molded plastic.

I’ll put it to you this way: If you’re in an environment hot enough to warp plastic, you shouldn’t even be there let alone a computer.

"Warning Level" temperatures:

Ambient temperature below 35 F/1.7 C: Generally speaking it’s too cold to operate at this point. You’re dangerously close to freezing and that’s when the physical properties of computer hardware change by flexing (usually). Not a good idea to operate a computer below this mark.

Ambient temperature above 90 F / 32.2 C: It would be rare to operate in this temperature because you’d be sweating profusely just sitting there, but some do. Your monitors and peripherals will run fine but the computer starts acting like an oven. Any air going thru there is also warm (or possibly hot) which at that point doesn’t help very much to cool it down.

Final notes

There will be those that vehemently disagree with me as to what’s too hot/cold for computer operating temperatures because I haven’t taken into account other factors such as altitude and humidity. And yes I know those both count in a big way. If you want to add commentary specifically concentrating on altitude/humidity, be my guest.

Temperature is very easy to dismiss because most of us don’t think about it when it comes to computers. We just assume it doesn’t matter when it fact it does. As long as you know when and when not to operate a computer based on temperature, you should be a-okay.

Also, bear in mind all computer hardware and laptops have specifications that state minimum and maximum operating temperatures - and they’re usually 100% accurate.