perhaps you should run virus scan and malware detector.
secondly, if this doesnt find anything you can try to diable 'PLL Overvoltage' in your bios. this setting protects your mobo from power spikes by smoothing out jumps in voltage, this can interfere with sleep/wake signals. if this solution doesnt work turn it back on immediately.
lastly, i dont really reccomend this but its worth a go: disable 'wake on lan' and/or all rpc services - this could be a wake signal reciever from your internet allowing an outside source to wake your pc from slumber without your physical input at the keyboard.