PC won't stay powered on

I've had my PC for at least 6 months now, and it has worked fine.

The other day, when I went to turn my PC on in the morning and it would power up for a few seconds (fans & hdd's whirring), before shutting down. It would automatically attempt a restart within a few seconds, but the same thing happens. I opened the case and had a look, nothing seemed out of place. I have tried it a few times since, and it can sometimes come on for a few seconds, and sometimes it will boot to windows and stay on for a few hours before shutting down again.

In a nutshell:

It powers up for intervals ranging from a few seconds to a few hours.
I haven't changed anything in the system.
I have tried a different socket & cable.

I have ordered an Arctic Power 500W to replace my stock 300W, but I'm not wholly convinced it will solve the problem. Anyone got any other ideas, I'll take any simple-stupid suggestions, in case I missed something obvious.

A common answer I have googled is CPU overheating - I highly doubt it is the CPU overheating in this case, due to it first occuring within a few seconds, and the PC staying on for a few hours at times.

I have noticed the USB which I use for my mouse freezing during the longer stints that it is working, which makes me worry that it is the mobo, but the problem occurs whether plugged directly into the mobo, or using the case USB at the front, so this may be a symptom of a larger problem.

Thanks for any help.
 
PSU fixed it, thanks for all your help everyone.

No seriously, I mean it, THANK YOU everyone who helped, bookmarking this page and checking it daily was a priviledge and not a pointless chore.
 
Well THANK YOU for the following:

1. Absolutely no system specs whatsoever given by you including the probably crap PSU you replaced, your OS, or security software. Diagnostic measures are so easy to do over the internet from one post.

2. For your stellar understanding that everyone here volunteers their time. We help as we can. Have you even stopped for one second to consider that perhaps you didn't get the help you needed is because the people who looked in a) either didn't have a solution or b) saw your post on the internet and looked in not being members but were looking for help themselves? I didn't think so.

3. Lastly, thanks for the tantrum.

Seriously, if I had seen your post, and I didn't, I would have asked you to do #1 and since you suspected the psu to use a multimeter or borrow one or purchase a digital one for a grand total of $20 and test your psu.

I would have also suggested that if you suspect heat might be the cause to open the side of your case and have a small fan blow air into your system on low.

* Now, this thread will probably be closed and I shan't look in on it anymore.
 
Nice one hero!

1/ I can't see how my problem needed system specs, it certainly didn't need an operating system mentioning.

2/ I think many things.

3/ No problem, made me feel better, and I'm glad I baited someone with it. Good little fishy.

4/ Of course everyone has a multimeter, I have 3 in my pocket whilst writing this post.

5/ I didn't suspect it was heat related, I ruled this out pretty quickly, shows how little you actually read of my initial post.

6/ Hopefully it will, it was no use to me, and will be no use to anyone in the future.
 
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