Why does my CHM file have a blank content pane?
So the problem is:
You open a CHM but you don’t see your content in the right-hand pane of the window. You see a browser message “Navigation to the webpage was canceled” or “Action canceled”.
This is due to Microsoft plugging security holes in Windows. This particular problem is either:
1. When you open a .CHM file over a network drive in a Windows XP/2003/Vista. This can be fixed by a simple registry tweak (see link)
2. Downloaded CHMs cannot be opened
After downloading, right-click the chm file, select properties, then click “Unblock”.
Microsoft Security Updates 896358 & 840315 block display of CHM file contents when opened from a network drive. And in Windows Vista, the security update is included by default. Microsoft issues workarounds (registry edits) for this problem in article KB896054. It documents the following methods to enable CHM functionality from a network drive.
Example 1: How to use UrlAllowList to enable specific URLs
Warning: Include only URLs to sites that you trust completely.
The following .reg file re-enables use of the InfoTech protocol to open remote content from the following locations:
• .chm files on \\productmanuals\helpfiles
• A Web application at the following URL:
Note You can paste the following text in a text editor such as Notepad. Then, you can save the file that uses the .reg file name extension.
Note As you can see from the example, to enable a UNC path to a network shared folder, you must add the following two entries:
You cannot use wildcard characters in the URL string of any site that is added to the UrlAllowList registry key. For example, the following string does not work:
However, the following string works:
This string lets the following sites serve content by using the InfoTech protocol:
You still cannot access .chm files by using a URL. Although we do not recommend that you do this, you can access the files by following example 2 and setting the "MaxAllowedZone" to three or larger. This is because the .chm file uses the Internet Explorer cache and every page that comes from the cache uses the Internet zone. Therefore, we highly recommend that you use the UNC path to access the Help files as previously described.
The Microsoft KB that describes the issue and shows severalways how to work around it is: