Switching PSU

did you wire it properly?

check your wiring, and if it still wont work swap out to your older psu, if it works, than your new one may be bad, but check your wiring to make sure.

there may be something faulty that caused the psu to blow. remove everything but bare minimum components (cpu, ram, motherboard, display) no hard drives or anything. then try it.

Was your original pc a dell or similar prebuild?
list what you can for us please partswise
you can try the paperclip test on the psu's, but that only shows it fires up, not how it holds up under load
Moto

could you list the specs of your pc, and how old is it, and as Moto said if its prebuilt?
What brand is your psu?

It is a prebuildt pc named "Master PC". Its got a pentium 4, an asrock motherboard, a geforce 7800 and a 300W psu.

I am trying to fix it for a friend. He came to because it suddenly couldnt boot anymore. The psu I am trying is a trust 420w psu.

Ok, thats not too bad then, Dull and some other builders swap wires around so you can't swap psu's,
Try the gfx card in another pc, to check it works,
try the psu powering same pc,
try putting one stick of the offending systems ram in the good pc (assuming same ram is used in both systems) and trying to boot, if it wont boot stick is bad obviously, check all sticks
might be worth reseating the heatsink depending how long its been on there as well but try the other stuff first to rule out them being the issue
Moto