Always suspect a recent security update before anything else.

For some time I've had the problem I sometimes can't open .chm's. I just get a missing page message no matter what book I click in the table of contents. I've been puzzled about it, but I haven't really thought about what could be the problem. Today, when I realized none of chm's seemed to work, I decided to ask google about it. At this point I wasn't even surprised when it turned out that some of the recent security updates from Microsoft had removed the ability to browse these from a networked resource. Duh! In order to solve the problem just add this to your registry:

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\HTMLHelp\1.x\ItssRestrictions]
"MaxAllowedZone"=dword:00000004

Lesson learned: Always suspect a recent security update, before anything else.