If you've ever downloaded a CHM documentation file and received a "Navigation to the webpage was canceled" message, you need to 'Unblock' the CHM file before opening it. This was a security improvement in Windows Vista, and there's no clear message that indicates that you need to jump through this hoop.

I saw a few reports of folks unable to open the Silverlight Toolkit's documentation file, and this is how to successfully open it. This is the same process for any CHM that you download - including, for instance, the excellent offline Silverlight SDK docs.

Here's what it looks like when your CHM file is not yet unblocked:

To fix this, simply right-click on the CHM file in explorer and select the 'Properties' menu item:

Then, click the 'Unblock' button near the bottom of the properties page:

You can then open the file and should see the first page in the file:

Hope this helps.