Why would a PC reset itself or shut down on its own?

Sometimes it is the capacitors on the mainboard are dying.
Check the mainboard if any of the capacitors are bulging.
If so, you need to replace the entire board (or solder on a replacement capacitor if you feel up to it).
Otherwise the capacitor might explode, and you do not want that.

Does it go through the shut down process or does it act as if the power plug was pulled?
Did you run a complete virus scan?
Try malwarebytes for malware.
Temperature issues? Download Real Temp to see if it is getting too hot.

@ragingloli I’ve had my PC reset on its own before, due to what you’re saying. Er, a mainboard IS a motherboard, yeah? Got that replaced and now it’s fine, but with this other one, it happens when you try to do specific things. Good idea though, it would probably be wise to look inside, either way.

@Charles Actually…once it went through the shutdown process, and started up again. When trying to download AVG, it just turned off, and as I said in my details, we had to fuck around with the plug for a bit to get it to turn on again. Two different things…And no we haven’t down a virus check either. Is it possible that by installing all those games, the former owner might have got viruses this way? There’s like 20 games on it.

No matter how many viruses you infect a system with, they can not prevent the system from turning on at all, so you can rule out any sort of virus related, or even software related causes for your problem. It is almost certainly a hardware problem, like the aforementioned capacitor issue, or the system refusing to turn on due to overheating (newer boards have a safety feature that automatically shuts off the system if the temperature gets to high and prevents it from starting until it dropped enough).

Ahh, so she should probably take this thing to a repair place then? I don’t think we’re going to be able to tell what’s wrong even by looking at the insides of it. And even so, I guess whatever is messing about needs to be replaced. Thanks for the help. I didn’t know viruses were incapable of preventing a system to not turn on. That’s a good lead, so far. (I thought root worms/kits/wtv could do that though?)

Rootkits can burrow themselves in the bootsectors of the harddrive and load with the OS, but the system itself only accesses the harddrive later in the boot process, so any rootkits can not have an effect on anything before that.
There are some viruses that can infect the bios itself, but I do not think that even those can prevent the system start, plus those are so rare that it is pretty much on the bottom of the list of possible causes.

My first suspicion is always overheating.

@jerv The case itself has like, no slits or anything anywhere, except in the back. How would one rectify overheating?

One at a time pull out then reseat the RAM modules.

Cleaning the fuzz out of the heat sink is important; I do that to mine every few weeks.

I also have one more case fan than I had when I bought my computer; I added a graphics card that creates a bit of heat and wanted to suck all the hot air out the back. (Always in the front and bottom, and out the back and top!) The last P4 system I had ran even hooter, especially with the graphics card I had in it, so it took four case fans to keep cool! One should be enough for you though.

Making sure that the heat sink is still making good contact with the CPU. Odds are that the thermal paste between them has dried up, so the CPU can no longer dump it’s heat into the heat sink for removal.

@dabbler Good point. All the heating and cooling over the years can un-seat anything in a slot like video cards or RAM.

Number one cause is usually overheating. Next would be viruses.

She brought it in to a repair shop, the one I go to when my PC messes up and I don’t know what to do.

Turns out the hardrive was all messed up, the power supply was on the verge of death AND…it had an unofficial copy of Windows installed on it. I did think it was pretty odd when I saw the ’‘page’’ for it whenever we turned on the PC. (or rather, when it reset itself) It’s like wow, never saw that before…the repair guy said those unofficial versions are really shoddy and even without the bad parts in the PC, we would have had trouble keeping the thing turned on for more than an hour. :/