Due to a security patch Windows has restricted access to a CHM. More info here: http://support.microsoft.com/kb/896358. The attached reg script fixes one of the 4 problems described in the KB. It works for most of the CHM files. Here is the file. Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\HTMLHelp\1.x\ItssRestrictions] "MaxAllowedZone"=dword:00000004