internal static class Api
{
	[StructLayout(LayoutKind.Sequential)]
	internal class LASTINPUTINFO
	{
		public LASTINPUTINFO()
		{
			cbSize = Marshal.SizeOf(this);
		}
 
		public int cbSize;
		public uint dwTime;
	}
 
	[DllImport("User32.dll")]
	public static extern bool GetLastInputInfo(LASTINPUTINFO plii);		
}

Unmanaged.Api.LASTINPUTINFO info = new Unmanaged.Api.LASTINPUTINFO();
TimeSpan sp = TimeSpan.FromMilliseconds(Environment.TickCount - info.dwTime);