If you mean that it hibernates after a long while in sleep mode, "Start" -> "Control Panel" -> "Power Options" like sminlal said. After that, click "change plan settings" for the power plan you have selected. Then click "change advanced power settings". Expand the "Sleep" section, then expand the "Hibernate after" subsection. Set that to a value of "0" (which the OS interprets as never). Click Ok. That should do it!