If you download a chm and it won't open (you get a popup warning about it's unknown origin, it's locked), here's what you can do:
A security update for Windows XP® may block access to CHM files
Windows XP® security update blocks active content in CHM files to protect your system security.
Solution: Run Windows Explorer, right-click on the CHM file, and select Properties from the popup menu. Click on the Unblock button immediately below the Advanced button on the General page. Click Apply to show the content. Once the CHM file has been unblocked, the Unblock button disappears.
Or it could be your local Internet Security settings are too restrictive.
Or you have a # in the folder's path name that the chm is in.
Or if the CHM is on a remote computer (ie. network drive).
1. Start 'regedit'.
2. Go to subkey:
3. If the key 'ItssRestrictions' doesn't exist under 1.x, create the key.
4. Right-click the ItssRestrictions subkey and create a new DWORD Value.
5. Name it 'MaxAllowedZone'.
6. Modify it's value to 1.
The different value options you can use are:
0 = Local Machine (only opens CHM files on your local hard drive)
1 = Local Intranet Zone (opens CHM's stored on a shared network drive)
2 = Trusted sites Zone (opens CHM's stored on a web site that is trusted)
3 = Internet Zone (opens CHM's from any website on the internet that isn't blocked)
4 = Restricted Sites Zone (opens CHM's from any website)
Under Windows 7, in addition to the registry hack, you will have to go into the Control Panel, open Internet Options, select Security tab, and in the Local Intranet zone, change the setting so that Automatically detect intranet network is off. Make sure the other 3 checkboxes are on.