Computer Info: Cleaning the inside of the Computer Case:

Start with spraying the fan or fans in the Power Supply, blow out all the accessible areas of the Power Supply, here is a picture of the fan on the back of the computer.
Next spray the heat sinks fan and fins the silver things under the fan, pictured below. You can use your flashlight to see the fins better and what needs more spraying. Sitting on top the CPU there is a heat sink and then on top of that is the fan. The heat sink sits right on top of the CPU and draws the heat up off the CPU into the fins, the fan pulls air up through the fins cooling them, this keeps the CPU operating at a cooler temperature. If the CPU overheats the computer will start rebooting or just start acting weird. If it gets way to hot it will ruin the CPU and the computer will not boot at all. So it is very important to make sure the spaces between those fins are cleaned out.

If you cannot get all the dust blown off the fan blades with the canned air, you can use a cotton swab to clean them. Once you have loosed up the dust use the can of air to blow off the excess.

Picture of a CPU, Heat Sink and Fan on a Mother Board that is about to be installed into a case. You can see the Fan sitting on those fins and the CPU is under those fins inserted into that white plate at the bottom laying right on the Mother Board.

Now take the can of compressed air and start spraying the inside of the case, use the flashlight to see if you are missing any dust. Make sure you spray the case fan on the lower right corner. Your case may have more fans then this case. If you cannot remove all the dust from these you can use a cotton swab on them like you may have done with the CPU fan. If there is some debris laying at the bottom of the case that will not blow out, try using the vacuum with the crevice tool attached to clean it. Just be very careful using the vacuum you do not want to use it on anything but the inside bottom of the case.

Make sure that you do not dislodge or unplug any wires inside the case, most are pretty hard to unplug. But there are some very small wires with small connectors that can come loose fairly easy, so be careful.
After you get the inside of the case cleaned, you may have air slots in the front of the case like these. Remember that fan I pointed out on the bottom right of the case, that pulls room air in through these slots and blows it over the parts in the case. The fan on the Power Supply pulls that air up through the Power Supply and blows it out into the room. This is how the inside of the case stays cool, if this air flow is restricted the parts in the computer will get to hot and they get very hot. When a part gets to hot you start get errors or reboots or the computer just starts acting weird, so cleaning is important.

Blow these slots out really good, a lot of dust and etc gets trapped in there, use your flashlight to see in those slots better. You can also see in there by looking in-between the fan blades of the fan on the other side inside the case. On one of my other cases there is a removable filter inserted into the bottom of the cream colored plastic shroud, if your's has that feature remove the filter and blow that out and then reinstall it. After everything is cleaned, touching the metal frame of the computer case with one hand, use the other hand to check and make sure all the plugs are seated tight and the cards are tightly seated into any slots snugly. Do not put to much pressure on anything, just push on them enough to make sure they are all seated tight.
There may be a Graphics Card installed in your computer, that has a fan on it. Make sure you blow this fan off to. It will be sitting in a slot on the Main Board and connected to the back of the case, it will be the card that your Monitor cable was connected to. Here is a picture of a Graphic Card and the fan.