Earlier my machine was not switching on. So i replaced my SMPS witha new one (ATX, 400W. After replacing, I am facing a problem w for the past few days. It restarts automatically after shut down. Ther is no problem while you work on it but this problem is shown only when i shut down my computer that too not always. Only on certain ocassions. I tried with another SMPS . But the result is same. Please advise me. Is it the problem with my motherboard?

You must have the drivers for your mboard loaded in order for it's main chipset's ACPI support and it's Shutdown and Hibernate, etc. to work properly. Whenever you load or re-load Windows from scratch, after Setup is finished, you must install the drivers for the mboard, particularly it's main chipset drivers, so that Windows has all the proper drivers for and information about your mboard. "Manufacturer/Model: ATX" doesn't tell us anything. Nearly all PC computers/mboards since the early 90's are ATX.

Also, if you downgraded from a higher watt power supply, maybe your computer does not have enough power. It may die when you start to put strain on the power supply. I would check what Tubesandwires suggested and also check your old power supply. It may not have had enough power for your system. "The biggest thing people over look when upgrading their computer is the Power Supply." Quote Leo Laporte

It is not the problem with any of my drivers . I didnt install my OS at all. It was working fine earlier. One day i heard a small sound form the back of my computer and my computer got dead. I replaced it with a new SMPS of the same watts and my system got working. But when i shut down , it automatically restart. That si the only problem. please advice me.

If the computer has no operating system, what were you doing when you were working on it? It sounds like the old PS failed some drastic way, such as it shorted, or a capacitor blew up, etc. When a PS fails like that, whether it damages the mboard or anything else connected to the mboard is, unfortunately, random. El-cheapo PSs are lot more likely to damage something other than themselves when they fail, but it can happen with any brand and model - it's just more likely if it is an el-cheapo. That brand and model is the old PS? You may or may not be able to see signs of damage (e.g. brownish or black/carbonny) .You will need good lighting and maybe a magnifying glass. Unplug the computer or otherwise remove the AC power to it. Look for any signs of something burnt on the surface of the mboard. Look for capacitors (larger cylinders) that have bulging or open tops rather than their being flat, and/or that fluid is leaking from, usually yellow or brown. Unplug the main 20 or 24 position and any other 4 position power connector from the PS to the mboard, examine the pins in the mboard sockets. Take out your ram modules and examine their contact edges and the contacts in the ram slots. Take out all your cards and examine their contact edges and the contacts in the slots they were in. Unplug the power connectors for your drives and examine them and the pins on the drives. Unplug the data cables and examine each connector that was connected and the pins on the drive. Unplug all the other wiring connectors to the mboard and examine them and the pins on the mboard. Unplug everything connected to the external mboard ports and examine their connectors and the port pins/contacts. If the mboard and it's components show no signs of damage, but something else does, don't connect anything that does show damage if you can, or use an undamaged connector or port or slot, try the computer. If you see any component (e.g. ram or a card) that shows signs of damage, don't plug it in if possible, try the computer. If you see damage in a ram slot or another slot or port, use another slot or port if possible, try the computer. You could also try clearing the cmos by moving a jumper on the mboard then moving it back about 5 minutes later with the AC power to the PS disconnected, but I doubt it would help.