Automatically reboots after about 2 seconds this is probably the problem: Wake on Lan (WOL).
You can disable it by booting to BIOS.
Go into the bios (set up) and make sure the tick for WOL (wake on LAN) is set to disable
Also look for the "start" on LAN or INFR setting to make them disabled.
You might also do this: but this is not always available on all machines...
Enable Advanced Power Management support
1. Click Start, click Control Panel, and then double-click Power Options.
2. Click the APM tab.
Note If you are using multiple monitors, the APM tab may not be available.
3. Select Enable Advanced Power Management Support, and then click OK.
No solution yet??
Try this:
Option 2:
Restart computer into BIOS,
1)go to Advanced in the Menu Bar,
2)select Power On Options in that list,
3)select "After Power Loss",
4)change ON to OFF,
move to File in the Menu Bar,
select "Save changes and Exit"
Your BIOS may be slightly different,
but you get the idea.