Question PC wont get to the bios, displays a glitched up screen.

[Wip_]

Hello,
I've recently plugged back an older computer of mine to give to my younger sister.
I plugged it in, try to reset the pc but windows wont let me, try to search for windows update but it tells me windows doesnt support the system or something like that. I reboot the pc and get a black screen, doesnt even let me access the BIOS. I reboot again and I get a glitched screen ( note that the patern sometimes changes ) while the fans ( cpu, case and psu ) are running normally.

I've tested the ram, the psu, refreshed the CMOS, verified all the cables and made sure everything is plugged in correctly.
I took the HDD and plugged it in my pc, cleaned it to then try to plug it back and install windows but the same screen shows everytime. sometimes my keyboard lights up but no matter what i try i cant get to the bios.
After trying to reboot the pc from the power button on the front, the screen is just black and this time the keyboard dosnt light up.
I'd guess the cpu is dead but the fact that it died just after rebooting is weird to me, any ideas ?

 

[rgd1101]

full system spec? include brand and model of the psu

 

[Wip_]

PSU : LDLC ATX500w 80plus Bronze
MB : MSI A88XM-E35 V2
RAM : G.Skill ddr3 2x4G F3-2133C9D -8GXL
CPU : no clue, I would have to remove the cooler, can do if neccessary
no GPU

 

[rgd1101]

so an apu. clean the cpu fan, could be overheating

 

[Wip_]

I'll give it a try. could it be the thermal paste was badly placed ( doubt it because replaced to change the motherboard by ldlc ) or just too old ? the computer is 5/6 years old.

 

[Wip_]

i've cleaned everything and changed the thermal paste. Still got the same issue. I also checked for any bent pins on the cpu but nothing.