Sorry , what does "PSU" Mean again in plain english ???
I'm sure you are meaning the Powersupply.
I guess I could go and check for it , but what would it matter ,
it would hardly be the cause for windows to abruptly reset.
I have recommended watage 450 more than enough I think
PSU = power supply unit, and wattage doesn't tell the whole story about power supplies, the amperage is important also especially the 12v rail. you said you reccently added stuffs in and maybe an instant of power comsumption created a shortage so the system shuts itself down, this is only my version of what might be your problem and to test that there's an option in window that tells what the window should do if there a power shortage, remain shut down or restart itself after power available. and if you choose remain shut down and when next time you got the same problem and then the computer remains shut down then the problem lies on the Power supply.