[SOLVED] Old PC only starts if 4-pin CPU connector is disconnected

[SharpeNoff]

Hello! This is the first time I post on this forum, sorry if I make any mistakes.
I recently found my old PC from 2007. It worked very well, I played a couple of older games and everything seemed to run fine.
I decided to completely disassemble the PC to clean it, as it was (unsurprisingly) dusty.

After I put everything back in, the PC doesn't start. The PS/2 keyboard's lights turn on when I flip the PSU switch on. When I press the power button, the CPU cooler runs for a second then the entire system turns off.
I take it apart again and only connect the necessary: CPU, PSU, power button, ram and pc beeper.
If I unplug the 4-pin connector that goes from the PSU to the MB, only then does the computer not instantly turn off.

There are no visibly broken capacitors or CPU pins. I tried reseating the CPU, plugging the 4-pin connecter the other way around, changing RAM, starting with no RAM to see if it beeps (it didn't). Since this worked perfectly yesterday, I either broke something while cleaning it or reinstalled something incorrectly.

Motherboard: ASRock 775i65G
CPU: Intel Pentium 4 3.00 GHz (with stock cooler)
GPU: AMD/ATI Radeon 9250
PSU: RPC 450WYS (450W)

P.S: The integrated GPU (the port from the motherboard) doesn't work. I am unaware whether the reason for it not working is the actual port itself or something with the CPU.
P.P.S: This is the first computer I completely disassembled and then reassembled, it's likely that I made a rookie mistake when reassembling it.

Edit: The 4 pin could be plugged in two ways, but only one of those ways made the cooler spin for a sec. The other way wouldn't do anything.
Here are some images, if you are interested.

 

[punkncat]

It could have been caused by handling, improper re-assembly, even in the way in which you cleaned the dust out....maybe a static charge. Really impossible to say.
Your 4 pin really should not have been easily able to plug in improperly. Not saying that is what damaged it initially but I am not certain having it backwards and turning out wouldn't have damaged something/else/further.

Being an obsolete machine anyway, take it for a learning experience.

 

[SharpeNoff]

It could have been caused by handling, improper re-assembly, even in the way in which you cleaned the dust out....maybe a static charge. Really impossible to say.
Your 4 pin really should not have been easily able to plug in improperly. Not saying that is what damaged it initially but I am not certain having it backwards and turning out wouldn't have damaged something/else/further.

Being an obsolete machine anyway, take it for a learning experience.

I agree with your points. Maybe in the future, the experience gained from doing this will help me.
I would still love to fix it, though, as long as it doesn't require a bunch of extra parts.

 

[SharpeNoff]

Update here: I tried using another PSU with the same wattage and it worked.