Is it Windows 7? If so, click start, and type power options, and select the first option that says change power saving options, and make sure its on High Performance (you may have to click show additional plans). If that isnt working, is the computer going to sleep mode itself or just the monitor? If the pc itself, make sure you are not pressing the sleep button on your computer as some keyboards may have a special function key that lets you put the pc on sleep with a press of a key.