You need to set your screen saver to <none>, how this is done varies slightly from one operating system to another so if you could tell we which version of Windows is on your PC I can tell you how to do it.
Right click anywhere on the desktop and select personalise, then click on the screen saver link in the bottom right, you can set the screen saver either to a longer time, or just select none as the screen saver to get rid of it totally. Then click on the link in the bottom of the screen saver window that says "change power settings" then click on the words "choose when to turn off the display" on the left and set the time to turn off the display either to a high number, or never.