First make sure that you have the required software for your Windows version.
Remove old versions first (If you have them):
Right click the desktop and click on gadgets in the popup menu.
Right click on all NetcamViewer Desktop gadgets and click on remove. Note: There can be multiple pages where you can find the NetcamViewer Desktop gadget. You can change pages by using the arrow's in the top left of the window.
Install the gadget as followed:
Extract the file from the downloaded zip-file.
Double click on NetcamViewer Desktop.
Click on Install when a window pops up to tell you that the publisher of the gadget can't be verified.