Let’s start with what qualifies as acting weird. If your computer starts to shut off with no warning, programs crash, you are able to see video artifacts, if it’s a laptop you might notice the keyboard or other surfaces get warm or even hot, you might hear the fans spinning at full speed or making strange sounds.
With a desktop, some things are pretty easy to look at. You might be able to see through a grill to the insides and look at the CPU or other fans in the case. Otherwise, the side of the case usually comes off with a couple of screws. If the CPU fan (a fan mounted about a metal heatsink on the processor) isn’t spinning, it needs to be replaced right away. Most modern processors are quite capable of getting hot enough to boil water without enough air flow through the heatsink. It’s also a pretty cheap and easy fix too. The last CPU fan I replaced was four screws and a plug to the motherboard that only goes one way. The hard part is getting the computer out from under the desk to be worked on. Fortunately, the owner did that before I got on-site.
Fans also can make noise. In normal operation, a fan should make the usual whirring sounds. If it’s making any grinding or clicking, it should be replaced. This happens when the bearings begin to fail. Then the fan will spin erratically, if at all. With the slowing in air speed, the computer will be unable to dissipate heat properly and that results in emergency shut offs, where you can have data loss or corruption of the OS if it happens enough times.
One thing that can be done to help the computer move enough air is keep a good margin of space around it, and in the case of laptops, under the computer. Laptops should only be placed on flat, solid surfaces. Avoid surfaces like blankets, pillows, and anything that might stop good air flow. With the tight placing of parts, laptops are difficult at the best of times to keep cool.
Over heating can also show in video artifacts. There can be oddly colored squares on the screen, or colors can appear to be “wrong.” If the computer has a discreet video card, it probably also has a fan on it. These, like the CPU and case fans, can fail. They most likely can be replaced too. It’s a little more difficult, as the fans are smaller and can be sometimes proprietary.
An enemy of good cooling is dust and animal hair in the case. This can be removed with a can of air or air compressor held about 18 to 24 inches from any part of the case, gently blowing the dust out of the case. This is best to do outside in good ventilation. Just unplug any cords and cables first, taking care to avoid hitting parts such as memory sticks, hard drives or other connectors.
Sometimes, having a laptop pad can help with heating issues. I’ve found that using little rubber furniture squares to raise the rear of the laptop up about a ¼ inch can help too. These can be picked up at just about any hardware store.
Even fans spinning at full speed can indicate that the computer is just working hard. In the case of displaying full screen video, you might hear the fans speed up, then slow down when the video program is stopped. Or if a program that requires lots of clock cycles starts up. Some computers will spin up the fans to full speed at boot, then slow them down when enough of the computer can tell the fans aren't needed at full speed. This is normal.
Your computer can also act weird if there is malware on it. This is your reminder to run your anti-malware and anti-virus software. Remember to update the programs also.