I have an application that I wrote that uses a web service to provide data back to a central app. I needed a windows sidebar gadget to consume the web service. However I noticed that when using javascript to consume the service I would get errors. I then realized that I had to enable my web service to allow for access via HTTP GET. This allows the web service URL to be accessed using a string with any parameters added to the string.
To enable this on your ASMX web service you just need a few parameters in the web.config. Inside the <System.web> settings.
<webServices>
<protocols>
<add name="HttpGet" />
</protocols>
</webServices>