If you click anywhere within the bryce window the render will stop, you have to minimize bryce by hovering your mouse at the top until the menu bar appears, then hit the minimize button. You can also use windows key + d or alt + tab. You may also have your cpu set to turn off when it goes into sleep mode. Right click on the desktop, click properties. In the Screen Saver tab there's a box that says Monitor Power, click the power button. A new dialogue comes up, go to the Power Schemes tab. There should be a menu with Turn off CPU after 3 min.....Never. It sounds like you have it on 3 min at the moment, change it to something higher.