Computer auto power on

Check the power supply switch and power supply section. Here is service manual for help.

Harman Kardon AVR 3500 Service Manual (Page 51 of 52) ' ManualsLib

I am assuming that you MSI MB is starting up as soon as you apply ac power from the wall....without pressing the start switch. This is one of three issues. The first can be solved in the BIOS. There is an option for auto-restart when power is lost and reestablished. This is normally done for unattended servers, not home PCs. The other possibility is that you have the Power Switch pins on the MB shorted resulting in a constant closed position. This will cause the same problem. The same issue can occur if the Power Switch is stuck resulting in a constant closure of the contacts. Disconnect the wires from the Power Switch pins on the MB. If the unit still autostarts, then the problem is either in the Power Supply or the MB.

The power supply depends more on what will be connected to the mother board, like what type of video card and how many devices (hard drives, CD/DVD drives, CPU AMD or INTEL, etc...) I've seen GAMING systems that needed 100watts just to power the video card... and 200watts just to power a SAN RAID 5 ARRAY card with 6 hard drives...

AVR = Automatic Voltage Regulation

It is not part of the computer. You buy it separately.

It protects your computer from power surges, thereby increasing the durability of that equipment overall.

change the power switch is just shorted closed

The red light is just a pilot light indicating that it is still
connected to the power line. To get it totally off, is to
unplug the unit from the outlet. If your PC is connected
to the AVR (Automatic Voltage Regulator), just turn the
power switch off at the AVR. Hope this help you.

hi buddy!

be sure that the power suppy can can handle all the components thier plug. and check the power button if there is no short circuit ok. also try the other power suppy if you encounter the same problem just reply i am willing to help you ok.

PS:
pls rate me if my solution solve the issue and you need more further information. but if is invaluable solution rate me fixya.

Hi,
If the front Light is flashing amber, the systems power supply has gone bad. Request you to replace the power supply and the front I/o panel and power switch . The issue would be resolved.

Thanks
mandeep

Something is wrong with the power supply.. there might be a blown out mosfet

if the power supply fan is working when you turn on the pc ,it probably is not the power supply,
and is most likely a motherboard problem ,if you have the ability to install another video card that you have (or a friend may have), this may solve your problem.

i am not sure what you mean when you say "my system unit automatically turn on after i turn on my AVR "
good luck
Terry