Ok, well if this PC isn't recently built, then it probably isn't likely that the motherboard has shorted out, however, if your friend wants to install a new graphics card, then you'll probably have to replace the motherboard anyway. But first, before I give you instructions on how to do that, we have to sort out your initial problem. From what I could see, system hang can occur for one of probably three reasons:
1. PSU can't cope with it now because of transistor ageing. What wattage is it?
2. Motherboard did get fried somewhere. Take a look for any burn marks and have a bit of a smell around. Find out if your friend had any power surges or smoke or anything coming from the computer.
3. CPU became overheated and fried, or something similar. Do you have another CPU to test with?
Whilst checking these, clean out all the collected dust in order to eliminate that as a possibility.
So if you could test all these three components separately in order to eliminate the problem, that would be great. Then get back to me. Now, in terms of getting a motherboard (as you asked where you should go to find one), it really depends what sort of CPU is currently in your machine, and if it is still current or if it is outdated. You will need to look for one with the same socket, or a socket that is backwardly compatible with your CPU. The socket is what the CPU sits in, and each generation of CPU has there own sockets. You can buy a motherboard from most computer stores. Once you have bought the motherboard, replacing it is basically a matter of unscrewing the screws in the old one and removing it (obviously, unplug all your hard drive cables, expansion cards, CPU, RAM, and PSU from it as well as case power buttons and anything like that). Then, plug back in all these things after the new motherboard is screwed in. If you need step by step instructions on where to find these things, let me know, however, I am assuming that if you're nearly A+ Certified that you should know what/where these are. If not, like I said, let me know. You will only need to replace your motherboard if a new graphics card does not fit the old motherboard. To find out what socket the old motherboard has, take a look in the manual or on the net, and look if it is PCI, PCI-Express, PCI-Express x16, AGP, etc.. And let me know for that too, as well as what sort of new graphics card you were looking for. Feel free to ask more questions here, or if you would find it easier, just ask me and I will give you my email address for help via Instant Messaging like MSN etc..
-Stephen
