Simple way to turn on/off 230V line power by a simple PC?
From what you have said, it sounds like you want a USB mains relay, This will allow you to switch a mains supply on and off from your computer, make sure the relay on the board you buy is rated to your countries mains power format (240V/110V etc..). Such devices can be easily found on ebay or other such sites.

Here is an example:-

http://www.ebay.co.uk/itm/New-1-Pcs-USB-Relay-1-Channel-Programmable-Computer-Control-For-Smart-Home-5V-/201072772365