To put this the simple way, it's dead.

We booted it up, it booted fine.

Installed a wireless NIC card.

Rebooted. Fine. Wouldn't connect to network. I sugestted maybe rebooting. It freezes at reboot. Turn of. Try again. Same, freeze at reboot. Turn of, turn on, followed by the sounds of the fans slowly running down.

Nothing. Dead. No life. Gone.

Power button does nothing.

Bought new power supply, nothing (though I might ad)
Bought new case (and so new power button/cables) seemingly, nothing - though I Must add that I connected them back up from memory, so there is a high chance the front conections will be wrong, untill I can get the Mobo manual out.

So, brainstorm?

I'm afraid this time it's gone forever... something died. Something died big.

Oh, I also tried removing the WNIC and attempting to boot, still no life. There is absolutely nothing.
What could cause a lifeless PC?

Things I've yet to check;

The power cord,
A new GFX card (She litteraly just had one)
A new CPU
A new Mobo

All of which either involve a new purchase or taking my PC apart - not a good idea with the luck we have

Suggestions?

Take it to a repair shop you trust, if such a thing exists.

Could be the cpu, or bios maybe. try disconnecting everything then just start plugging stuff in.

Well, we found the mobo's manual, I'd pluged things in to the right place anyway aparently.

Still, I've been fiddling today, we've got it to boot. Must have been a mix of faulty case wiring and me plugging something in wrong.

Oddly though, I plugged in the old PSU, it fire up. Turned it off, then it wouldn't turn back on. So I replugged in the new PSU, turned it on fine.

Then, I put the old one back in (it has more connections) and it booted fine. Again and again.

We'll see.

You may have some sort of short somewhere. A PSU will typically refuse to boot if it senses a short. Damned if I know where, though... not alot to go by, unfortuanetly. You really have bad luck.

Look at the bright, there is enough cable inside the box to hang yourself