Resolved iMac Late 2009 shuts down randomly when running Windows

[NickArchiver52]

iMac Unibody, Late 2009, 21.5inch, 3.06ghz C2D, ATI Radeon HD 4670

I have it running Mac OS X Yosemite just fine, able to use it & leave it for hours while it backs up to my Time Capsule, no random shutdowns. But when I boot into Windows, it either shuts down immediately after leaving the boot picker, or it runs for a few minutes then shuts down. I don't think it's the PSU or thermals as I can run it in Mac OS just fine, and I've repasted both CPU & GPU with Artic MX-4.

Edit: forgot to mention I tried both Windows XP and 7. Same issue

What could be going on here? Is this a software or hardware issue?

 

[DeltaMac]

I am guessing that it shuts down because of something overheating, with Windows using some part of the chipset that macOS is not affecting quite so much.
Are you still running the (maybe) original HDD - or, has the spinning rust been replaced with an SSD?

 

[Nermal]

When you say "shuts down", do you mean it goes through a shutdown process (closing all running apps etc.) or does it instantly power off?

 

[NickArchiver52]

When you say "shuts down", do you mean it goes through a shutdown process (closing all running apps etc.) or does it instantly power off?

Instantly shuts off

 

[NickArchiver52]

I am guessing that it shuts down because of something overheating, with Windows using some part of the chipset that macOS is not affecting quite so much.
Are you still running the (maybe) original HDD - or, has the spinning rust been replaced with an SSD?

I'm using it with a HDD, it's not original, good health. I don't have any plan to replace it with a SSD unless it is causing the problem.

 

[Wowfunhappy]

Instantly shuts off

Sounds to me like a PSU issue. For whatever reason, the PSU is no longer able to provide enough power to run the computer. And for some reason, Windows uses more power than macOS.

I don't have any plan to replace it with a SSD unless it is causing the problem.

I don't think it's causing the problem per se. However, HDDs use more power than SSDs, so switching to an SSD may help you stay under the power limit of your weakened PSU.

Mind, this is all very speculative!

 

[NickArchiver52]

Okay, I've swapped the PSU from a 2011 parts machine (I didn't do it earlier because I didn't want to dissasembleit again.) Appears to work fine now. Thanks guys!