I just reinstalled Windows 7 after the original factory installed operating system became corrupted. Now, when I haven't been using my computer (a Dell desktop) for a few hours, it doesn't just go into sleep mode, it shuts down, and I have to switch it on (but I don't have to log in). This never happened with the previous Windows 7 that came with the computer. Is there a setting I can switch to keep the computer from shutting down automatically?