System starts up after shutdown

hi i have a problem with my home built machine, which i have had for like a year and a half i think.

whenever i software shut down the machine after about 2 seconds it gets a start up signal which turns it back on. if i hold the power button for a few seconds and force it down it is fine.

does anyone know why this could be happening? thanks.

this is my hardware
-Intel E8400 Core 2 Duo Wolfdale @ 3.0ghz
-Intel BLKDP35DPM (also called the DP35DP) LGA 775 Intel P35 ATX Intel Motherboard - OEM
-RAIDMAX Elite ATX-208 Beige 0.7mm SECC ATX Mid Tower Computer Case
-MSI NX8600GT-T2D256E OC GeForce 8600GT 256MB 128-bit GDDR3 PCI Express x16 SLI Supported Video Card
-MSI PC60G 32bit PCI2.2 Turbo G Wireless Adapter
-Seagate Barracuda 7200.10 ST3250410AS 250GB 7200 RPM SATA 3.0Gb/s Hard Drive
-Antec earthwatts EA500 ATX12V v2.0 500W Power Supply
-SAMSUNG 20X DVD±R DVD Burner Black IDE Model SH-S202J
-OCZ Vista Upgrade 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model OCZ2VU8002GK
4 answers Last reply
More about system starts shutdown
  1. 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.
  2. nope i select shutdown and the machine comes back up and does a normal clean boot. i once tried it with a linux live disk im pretty sure. ill try it again just to be sure.
  3. 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.

    any ideas?
  4. 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.
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