posted 10 years ago
In Windows, the CHM extension usually corresponds to the Compiled HTML Help file format. On my Windows 7 it is associated with C:\Windows\hh.exe, which is described as "Microsoft HTML Help Executable".
Windows marks files that were downloaded from the internet and stored on NTFS partitions with special attribute as unsafe, and usually provides one additional warning when opening such a file. It seems that this mechanism doesn't work properly with CHM files and they need to be "unblocked" manually, as described in the link I've posted above.