PC turns on for 1 second then shuts down with blinking power light.

Jan 9, 2019
Well hello, I've read multiple questions here but none of these answers work for me. I will try to be very detailed so please bare with me.

The story first:
I play mostly trackmania with this old win xp pc and always overclocked it (it was 99% percent of the time stable and the temps were good) all that was a perfect thing after i discovered a video setting in the game for the fps lock (normally at 60fps) and i set it to 144fps because i sometimes got that many. however, i started the game and my pc immediately shut down without a beep or anything. i tried to start it again but it only powers on for 1 sec (including fans) and then turns off (also including fans) and the powerlight starts blinking on and off repeatedly.
I already bought a new PSU which is the Thermaltake smart 500W RBG PSU. I also bought a new CPU, new RAM, new CMOS battery, freed everything of possible dust, but nothing happens. always the same thing. No compensators are blown or something like that, they all look like they're in a good shape. Also the Motherboard nowhere has a darkish colour because of the overclock.
Here's a list of the old parts:

-AMD Sempron3000+ @2.15Ghz (Normally @1.8 Ghz)

-Standard VGA GPU (Onboard GFX)

-300W PSU

-DDR400 PC3200 RAM (2x=1gb)

-MSI 7181 Ver. 2 (Socket754) Mobo

-Standard AMD Cooling fan

-4 Case Fans to make sure everything stays cool

-TV as Monitor

Here the parts I bought and tried:

-AMD Athlon 64 3400+ Newcastle @2.4GHz (normal)

-DDR400 PC3200 (2x=2gb)

-500W Thermaltake Smart RGB PSU

-New CMOS battery (3V)

-Radeon HD 3850 512mb DDR3 AGP-Slot GPU

Please help me as this is important to me. The User's manual says:

"MSI Reminds You. There is a mechanism of this mainboard to protect it from being damaged. The power will shut down automatically in two conditions: the temperature of CPU reaches 100°C, or the low voltage occurs during booting up. Please follow the instructions below for this issue 1. The power LED will blink continously. You should unplug the power cord or turn off the power switch 2. After the power LED stop blinking, plug on the power cord or turn on the power switch, then you can reboot your system again"

But no matter how often I try, it doesn't Work.