Prevent windows from going into sleep when my program is running?

set wsc = CreateObject("WScript.Shell")

Do WScript.Sleep (60*1000) wsc.SendKeys ("{SCROLLLOCK 2}") Loop

-put the above code in notepad and save the file as .vbs and double click the file

Using PowerCreateRequest, PowerSetRequest, and PowerClearRequest functions is the preferred method. Details and sample code (C/C#) are inside http://msdn.microsoft.com/en-us/library/windows/hardware/gg463205.aspx

After considering vim's answer

"Using PowerCreateRequest, PowerSetRequest, and PowerClearRequest functions is the preferred method."

with the linked AvailabilityRequests.docx on msdn which is exhausting to get into it (too much to read), I have searched the web for a concrete example in c# that is based on the PowerCreateRequest and found http://go4answers.webhost4life.com/Example/problem-monitor-wakeup-service-windows7-12092.aspx [EDIT 2016 - isn't available anymore]

Copied and adapted it to my needs (PInvoke of CloseHandle copied from msdn):

using System.Runtime.InteropServices;

    #region prevent screensaver, display dimming and automatically sleeping
    POWER_REQUEST_CONTEXT _PowerRequestContext;
    IntPtr _PowerRequest; //HANDLE

    // Availability Request Functions
    [DllImport("kernel32.dll")]
    static extern IntPtr PowerCreateRequest(ref POWER_REQUEST_CONTEXT Context);

    [DllImport("kernel32.dll")]
    static extern bool PowerSetRequest(IntPtr PowerRequestHandle, PowerRequestType RequestType);

    [DllImport("kernel32.dll")]
    static extern bool PowerClearRequest(IntPtr PowerRequestHandle, PowerRequestType RequestType);

    [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true, ExactSpelling = true)]
    internal static extern int CloseHandle(IntPtr hObject);

    // Availablity Request Enumerations and Constants
    enum PowerRequestType
    {
        PowerRequestDisplayRequired = 0,
        PowerRequestSystemRequired,
        PowerRequestAwayModeRequired,
        PowerRequestMaximum
    }

    const int POWER_REQUEST_CONTEXT_VERSION = 0;
    const int POWER_REQUEST_CONTEXT_SIMPLE_STRING = 0x1;
    const int POWER_REQUEST_CONTEXT_DETAILED_STRING = 0x2;

    // Availablity Request Structures
    // Note:  Windows defines the POWER_REQUEST_CONTEXT structure with an
    // internal union of SimpleReasonString and Detailed information.
    // To avoid runtime interop issues, this version of 
    // POWER_REQUEST_CONTEXT only supports SimpleReasonString.  
    // To use the detailed information,
    // define the PowerCreateRequest function with the first 
    // parameter of type POWER_REQUEST_CONTEXT_DETAILED.
    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
    public struct POWER_REQUEST_CONTEXT
    {
        public UInt32 Version;
        public UInt32 Flags;
        [MarshalAs(UnmanagedType.LPWStr)]
        public string
            SimpleReasonString;
    }

    [StructLayout(LayoutKind.Sequential)]
    public struct PowerRequestContextDetailedInformation
    {
        public IntPtr LocalizedReasonModule;
        public UInt32 LocalizedReasonId;
        public UInt32 ReasonStringCount;
        [MarshalAs(UnmanagedType.LPWStr)]
        public string[] ReasonStrings;
    }

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
    public struct POWER_REQUEST_CONTEXT_DETAILED
    {
        public UInt32 Version;
        public UInt32 Flags;
        public PowerRequestContextDetailedInformation DetailedInformation;
    }
    #endregion



    /// <summary>
    /// Prevent screensaver, display dimming and power saving. This function wraps PInvokes on Win32 API. 
    /// </summary>
    /// <param name="enableConstantDisplayAndPower">True to get a constant display and power - False to clear the settings</param>
    private void EnableConstantDisplayAndPower(bool enableConstantDisplayAndPower)
    {
        if (enableConstantDisplayAndPower)
        {
            // Set up the diagnostic string
            _PowerRequestContext.Version = POWER_REQUEST_CONTEXT_VERSION;
            _PowerRequestContext.Flags = POWER_REQUEST_CONTEXT_SIMPLE_STRING;
            _PowerRequestContext.SimpleReasonString = "Continuous measurement"; // your reason for changing the power settings;

            // Create the request, get a handle
            _PowerRequest = PowerCreateRequest(ref _PowerRequestContext);

            // Set the request
            PowerSetRequest(_PowerRequest, PowerRequestType.PowerRequestSystemRequired);
            PowerSetRequest(_PowerRequest, PowerRequestType.PowerRequestDisplayRequired);
        }
        else
        {
            // Clear the request
            PowerClearRequest(_PowerRequest, PowerRequestType.PowerRequestSystemRequired);
            PowerClearRequest(_PowerRequest, PowerRequestType.PowerRequestDisplayRequired);

            CloseHandle(_PowerRequest);
        }
    }

I had a problem like this with a hardware device connected via usb. XP /Vista would sleep/hibernate right in the middle of ... Great you say, when it resumes it can just continue. If the hardware is still connected!!! Users have the habit of pulling cables out whenever they feel like it.

You need to handle XP and Vista

Under XP trap the WM_POWERBROADCAST and look for the PBT_APMQUERYSUSPEND wparam.

   // See if bit 1 is set, this means that you can send a deny while we are busy
   if (message.LParam & 0x1)
   {
      // send the deny message
      return BROADCAST_QUERY_DENY;
   } // if
   else
   {
      return TRUE;
   } // else

Under Vista use SetThreadExecutionState like this

// try this for vista, it will fail on XP
if (SetThreadExecutionState(ES_CONTINUOUS | ES_SYSTEM_REQUIRED | ES_AWAYMODE_REQUIRED) == NULL)
{
   // try XP variant as well just to make sure 
   SetThreadExecutionState(ES_CONTINUOUS | ES_SYSTEM_REQUIRED);
}  // if 

and when you app has finished set it back to normal

// set state back to normal
SetThreadExecutionState(ES_CONTINUOUS);