Originally Posted by NooNoo
Let's talk about the power supply (psu) first.
Just because a psu supplies some electricity to the motherboard for it to apparently start, doesn't mean it's working properly. A psu takes in voltage from your socket in your house - 110, 220 or whatever depending on where you live... it then converts that power into 12v and 5volt outputs with a given amount of amps/watts. If the power supply is failing, it may not output the voltage and watts in a steady reliable stream but may under supply or over supply.
For a non technician, the only way to test this is to replace the psu with a similarly rated new one.
The process on how to tackle taking apart your computer is reasonably simple. You need a bench to work on with power you can turn off. You need a little pot to put the screws in or if you are worried a sheet of paper and sticky tape. You tape the screw to the paper and write next to it where it came from.
First challenge is taking the case apart. If it's a branded computer, it could be a lot of fun figuring out the release mechanisms or getting the right screwdriver end (star/torx) to fit. Standard computer cases of this age were towers, had 4 screws in the back and both sides and top of the case are pulled back slightly then lifted off. Some had the panel on the sides that could be removed separately.
So what have you got ?