When you say "Software shutdown", are you clicking the arrow next to "Sleep" and "Lock" to open up a window and then clicking "Shut Down"
If you are using the "Sleep" option, your "Advanced Power Settings" may combine with BIOS/Device settings to immediately wake the system up when, say, the mouse is moved. Or something less obvious causes the system to wake up.
yup i just booted a live cd right now and did 'shutdown -h now' so it definetly must have done a full halt. the system turns off,and fans stop, but like 2 seconds later it boots back up.
i have no idea what could be causing this. since it happens in windows and linux i would think it was a hardware issue but since it does shutdown with a force shutdown by holding the power button for a few seconds im not sure.
Try removing the motherboard connectors from your Power Switch (and Reset Switch if you have one). Then start the PC by placing a screwdriver momentarily across the two pins your Power Switch was connected to. Then do a software shutdown.
This should not fix your problem; if it does, the problem is in your switch(es) or the connector(s). That eliminates the switches.
Clear CMOS, restore defaults, and while resetting what you need check your BIOS for any settings that have to do with waking up the PC. Make sure nothing does. That eliminates BIOS.
If you have another psu, swap it in. Your current psu could be thinking its been asked to power up when it has not.
If you have any (case) device that is powered by standby power in order to wake the machine up - like a remote control - disable it.
You could reassemble the system outside the case . . . use the screwdriver trick to start it uup . . . but I can't even speculate what that might fix lol. Unless some kind of electrical contact is telling the psu to fire up.