Brand new PSU, the kid told me before he switched it out, he noticed the old PSU that came with the case acting the same way, so I told him to go get the old one while I look into this.. thing is, it worked perfectly fine after I put it together, installed the OS, installed drivers, rebooted several times, worked absolutely fine till he called me the day after and said it wasn't powering up.

I'll check the mobo regardless.