Computer won't standby.?

I had my computer for many years now and i almost always leave in on standby when i go to cook or for a time period that is not too long but i dont use the computer meanwhile. I used to either selest start>turn off computer>standby or i set up so after 20 or so minutes the computer stands by itself through power management. The latter being the most common. However just one random day my computer refused to do so - when i standby using any of these two methods, my computer will literally restart and after a while it will leave me to the logon screen which u usually get when u wake up from standby. The effect is almost similar to when u standby it and then inputting with the keyboard to wake it from standby. Can anyone explain why is this? Thanx

This computer has not been setup with wake on LAN or anything like that. New software... cant really remember but i doubt i installed something.. Maybe it is the motherboard driver? I reinstalled my system but not sure the dates..