First, you have to enable Windows to show the file extensions. Go "Start" --> "Run" and type in control folders. Click "OK". Click on the "View" tab and uncheck the "Hide extensions from known file types" option. Click "OK". Locate the downloaded gadget file and rename the file to Memory Gauge.rar. Extract them and rename the extracted folder to Memory Gauge.Gadget. Then, move the folder to C:\Program Files\Windows Sidebar\Gadgets\. Launch Windows Sidebar and open the Gadget Gallery. Now, you might be able to add the newly installed gadget onto your sidebar.