In the BIOS there is a setting called 'wake on lan' or something similar. When a network packet addressed to the PC arrives and the PC is powered down and 'wake on lan' is set then the PC powers up. See if this is the problem.
Also, certain fails in Windows shutdown cause a re-boot. There is a windows setting for this. If the PC never boots randomly but always boots exactly 3 secs after shutdown, try start menu then right click my computer and click properties.
click on advanced tab and then under startup and recovery click on settings..
then look at system failure uncheck the automatically restart