2006-01-28: A small but important correction on step 2 added.
For those of us who don't like to read manuals, dig architectures, etc. but rather learn by getting dirty with code, below you will find the four steps to your first (very silly) Vista Sidebar gadget:1. Create an HTML file with Notepad by the name of MyFirstGadget.html, with this contents:
<title>My first gadget</title>
<span id="gadgetContent" style='font-family: Tahoma; font-size: 10pt;'>¡Hola mundo!</span>
2. Create an XML file with by the name of gadget.xml (must be named exactly like this!) with this contents:
2006-01-28: Added a closing slash on the info tag
<?xml version="1.0" encoding="utf-8" ?>
<name>My first gadget</name>
<author name="Edgar Sánchez">
<info url="http://www.logicstudio.net">http://www.logicstudio.net" />
<description>A very silly gadget (you've got to start somewhere, don't you?)</description>
<base type="HTML" apiVersion="1.0.0" src="MyFirstGadget.html" />
<platform minPlatformVersion="0.3" />
3. Zip those two files in one archive named MyFirstInstaller.gadget. Do note that the file extension must be .gadget not .zip nor .gadget.zip nor anything like that, just .gadget.
4. Double-click on MyFirstInstaller.gadget and click on the Install button on the dialog box that pops up.