Turning on psu turns on pc

I'm building my own PC for the first time, but when I run it, something weird happens.

When I flip the PSU switch to be on, it immediately gives power to everything before I the power to the motherboard is activated.

However, nothing actually shows up on the screen until I activate the normal power. When I do it loads OS, everything becomes normal...
When I click Shut Down, it does shut down - but all my fans are still spinning, and it seems like they're receiving power when they shouldn't.

Someone told me that the problem might be caused by a BIOS setting that automatically turns on the computer when there's power, and that setting should be off.

I went into the BIOS to check that setting, I think I found it but it was already on 'off'

It was called "AC Back Function"

So that must not be it as it was already off.

What is the problem, and how can I fix it?

 

Re-check all your connections

 

I took everything out of the case for an external build, same problem as in the case.

I can check another PSU and see what happens though...

 

So I tested my friend's PSU just now on my external build (its easier to change hardware stuff on an external build) and it works fine.

Perhaps I have a faulty PSU?

My PSU orignally worked, but then it started doing that weird thing where it would power the motherboard before I pressed power. (I still had to power it to actually boot up the pc)

My power supply is Rosewill Green Series 530 watts

When it worked, it was my external test build. When I put everything in the case and ran it was when the PSU acted weirdly.

But doing another external build had the same results as the internal build.

Could the PSU have gotten messed up when I put it in the case?
Or did the PSU just happen to be a faulty one.

 

So its more likely a faulty PSU than me breaking/messing it up? I suppose I could return the Rosewill and get a better psu.

And although that Corsair one looks like a nice PSU... its really expensive. I want a PSU that can power my parts (Intel Core i5-2500K, Radeon HD 6850 - I don't plan to ever Crossfire) decently for a cheap cost, no more than $60 (maybe a bit more if its really a good PSU)

 

I'm sorry, but I don't understand your philosophy. The current price of an i5-2500K is about $225. The current price for your graphics card varies from $130-$150.

The Rosewill power supply you are currently using is about $55 and has a 2 year limted warranty. The Corsair Enthusiast Series TX650 is $85.00 (after rebate) and has a 5 year warranty. You use a fairly expensive processor and graphics card the combination of which would have a cost of approximately $350-$360, but you want to hold the price of a power supply to around $60 or approximately 1/6th (about 16%) the cost of the processor and card.

About 25% of all hardware related computer problems are caused by power supplies. I understand this is your first build, but you will learn to NEVER cut corners with power supplies.