Computer turns on by itself

Hello,  
I have a problem with HP DC7800. The computer won't stay turned on when i turn him off, put him to sleep or put him to hibernate. It simply turns on after 3 seconds without me doing anything. The only thing i can do is unplug it from electricity.
I tried clearing CMOS but that didn't solve the issue. When i press F10 to get into BIOS, it says "entering setup" but then instead of actualy going to BIOS, it just continues with the boot, nothing i do will get me into BIOS.