Are you using a dialup modem?
If so, your bios might be set up to "wake up" the computer if a modem signal is detected.
run the BIOS settings software ( either press and hold delete or F12 on reboot before anything shows on the screen) or refer to your P.C manual to check this setting.
The menu for this should be in the power saving section.
hope this helps, if not it'll eliminate one possible cause!!
You are effectively leaving your computer on standby and therefore still using electricity even though you say the computer is "off". I recommend that you switch off all electrical appliances at the wall when you have finished using them.