Alienware HPC-420-302 DF power supply died

Old power supply? Sounds to me like the power supply is sending a faulty signal to the motherboard, thus causing the computer to try to correct the problem with a re-boot. My Alienware is bad for that when the power supple gets weak. Buying a new power supply always corrects it.

I had to deal with the new power supply. The problem is the went from a two contact plug to a tree connector. They made the power supply with a little smarts. The third pin was serial memory in the that tells the computer what size power supply is connected to the computer. If the power supply is a smaller power supply than came with. At one time they a different size connector for different size power supply. So you can connect a smaller power supply to a larger computer. You can burn out the power supply. The one draw back was if the memory chip died the computer would think the power supply was too small or a third party power supply not made for the computer. I've has two power supply where the chip died and the computer would not charge. I had other computer that use the same power supply and the computer would startup and charge the battery. Also if the computer thinks it had a smaller power supply the computer cut the cpu speed.
I don't know why if it has smaller power supply why it could not just charge the battery and run the computer from the battery.
If can find another power supply to try to see if computer run at all will run.
Another problem was the power supply pin sticks out the side and is easy to hit and break the solder connector contact on the inside.
You need someone that can take computer a part enough to see if the power supply connector was lose on the board.
Volt meter the power. Outside ground, inside +, center pin is the ID pin or memory pin.

I hope this help. I like old power supply. They were simple.

If your desktop computer went bang from the back of the computer and the burnt smell comes from the fan grill of the power supply then the switch mode power supply definitely failed. The bang and burning smell was probably one or more of the capacitors that has exploded inside the power supply.
You can replace the switch mode power supply, make sure you purchase one that has the same total wattage or higher wattage.
There is a chance that when the power supply failed it may have caused a voltage spike that could have damaged some other components. You won't know until you replace the faulty power supply.

most power supply units will fit you can get a larger wattage output eg: heavy duty if you have added any more devices to your computer more RAM,a larger hard drive,more cd/dvd drives these added will put a greater need for wattage output on the PSU
you should check all the electrical connections before you make a decision and make sure they are the same as as the ones that you will need to run your computer also
check the measurements inside the frame to make absolutely sure it will fit
you can get lots of PSU

Take your desktop computer (and the dead power-supply) to any computer-store. They can sell you a "generic" power-supply that will be compatible with your original power-supply.

Hint: the smaller the computer-store that you go to, namely a "mom-and-pop" company, the more-likely that they will install it for free, if you buy the new power-supply from them.

The switch mode power supply in the desktop computer has failed. Most likely the capacitors in the power has exploded.
You need to open the desktop computer and remove the old power supply and replace it with a new one. Take the old power supply with you to your friendly computer shop and get a replacement power supply with the same wattage or greater wattage.

You can get the fan from any other power supply. Try a computer repair shop.

Please don't forget to rate if the answer helps

The dust could cause the burning smell, if the smell has an electrical smell then most likely the capacitors are leaking in the power supply and about to blow/explode.
Is the power supply come from a desktop computer? If so all ATX switch mode power supplies have standard mounting holes and power connectors.
Take out the old switch mode power supply and take it with you to your friendly computer shop and compare it with a new one. Make sure the wattage rating of the new switch mode power supply is the same or greater than the old switch mode power supply.

The problem may cause by the defective transformer on Power supply board. there are 3 transformers on the power supply. http://www.lcdparts.net/LIPSInverterDetail.aspx?InverterOriginal=4H.B0700.051
Please do the vision inspection on power supply, you should be able see the obvious burn on one of these three big transformers.