Microsoft in their infinite wisdom released a security update years ago that prevented users from viewing HTML Help files over a network. I’ve seen instructions for fixing this capability in several places online, but with slightly different variations. Most of the instructions assumed that you’d be using UNCs to access the network rather than Windows drive letters, so some of the instructions threw me. After beating my head on the problem for a few hours, this is what worked for us.
1. Open a registry editor and go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\HTMLHelp\1.x\ItssRestrictions. If ItssRestrictions does not exist, right-click the 1.x key, click New->Key, and name the new key ItssRestrictions.
2. Click ItssRestrictions to select it. Click an empty area in the right-hand portion of the window and click New->DWORD value. Name the entry MaxAllowedZone. Right-click MaxAllowedZone and click Modify. Set the value to 1.
3. Again, click an empty area in the right-hand portion of the window and click New->String Value. Name the entry UrlAllowList. Right-click UrlAllowList and click modify. Enter the path to the .chm file you want to open, followed by a semicolon. Next, type “file://” followed by the path to the .chm file again. For example, I have a help filed called trashflo.chm saved in the directory G:\TFManual. My entry for the UrlAllowList looks like this:
If you need to access more than one help file, just add them to this entry, placing a semicolon between each.
[Note: If you’re using Universal Naming Conventions (UNCs) instead of mapped Windows drives for your shares, the entry would look something like this: