Don't Sleep is a simple tool that will prevent your system from powering down, sleeping, hibernating, restarting and more.
Why bother? You could, of course, set up your PC to make sure this never happens, by setting the appropriate power settings. But this isn't always convenient. Most of the time you might be happy to have your system shut down after it's been idle for 5 minutes, say. If you occasionally need to use it to run presentations, though, then this could easily become annoying.
When you need to be sure that your PC stays on, then, just run Don't Sleep. The program will automatically prevent your PC from going into standby, hibernation, shutting down, launching the screensaver and more (though clearing a few boxes will allow you to configure exactly what it'll block, if you prefer).
You can also set the program to work for only a given period of time. So if you want to be sure the system will stay running until you get back from lunch, say, maybe because it's downloading a file or doing something else important, then simply set your return time, check "Use Timer" and you're ready to go.
A "To Tray" button then allows you to minimise the program to the system tray, so it doesn't waste taskbar space.
Or, if you'd like to tweak your Windows settings directly, then Don't Sleep's System menu provides easy access to your power plans, display settings, screensaver timeout and more.