+1 for RMA
When you un-overclocked, did you clear CMOS/load defaults or just remove the overclock via settings? It's usually best to reset to defaults and re-apply the settings you prefer than potentially forgetting to reset something such as voltage.
I don't think it's a coincidence that it died shortly after un-overclocking it. Sounds like you missed a setting. If you lost a VRM, I think the chances of your CPU being fried are good. If you have a bit of time, I'd take the motherboard out and look over it for any scorch marks.
If you suspect the PSU, give it a sniff. You can test it without a load by unplugging everything, taking a wire/paperclip and shorting out the green wire to a black wire on the 20/24-pin ATX power connector. Once the PSU is running, check voltages with a multimeter. This isn't ideal since what we really care about is voltages under load, but it's a good start. Also, it's generally not recommended to keep a PSU running with no load for very long.
If a PC shuts down in the first few seconds on boot, it'd be a bad idea turning it back on without checking your heatsinks.
Good luck getting it fixed though. The last system I had explode on me was my old Athlon XP rig. The power supply went and took out the motherboard and CPU. Had to fall back on a P3 Celeron eMachine and it was terrible.
Any computer, no matter how old, can serve a meaningful purpose.
Behind every closet door is a dusty computer waiting to be reborn.