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I downloaded a CHM file. When I double click it it prompts me to open / save / cancel. Whether I open or save a new copy, the 'new' version will prompt the same open / save / cancel popup ad infinitum.
Searching google it seems that Microsoft have deemed it right for security reason to block these files by default. The solution is to right click the file, and click the 'unblock' button at the bottom:

enter image description here

The problem is that I don't have that button on my system:

enter image description here

BTW, the button is also missing for .exe files.
I'm using Win7 64bit. Any ideas?

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See here for how to disable this annoying security feature, and here for how to bulk unblock from the command line. – Karan Nov 10 '12 at 14:55
Will this work for XP? I have the same issue on my work machine. – Yoav Nov 13 '12 at 13:28
Yes, both should work on XP as well. – Karan Nov 14 '12 at 0:15
A method using Windows PowerShell was provided in this question:… – DaveInCaz Jun 29 '15 at 14:59
Possible duplicate of How to Unblock Previously Unblocked CHM File – jgauffin May 17 at 9:14

Same thing happened on mine.

Open Regedit


Create subkey ItssRestrictions

Create DWORD MaxAllowedZone with value 2

This should fix it.

If not working. Move the file to different disk. And reopen it to see.

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worked for me thx! – kofifus Jan 28 at 3:16

CHM files are only blockable (and thus unblockable) on NTFS filesystems. Probably all other sources of files (except network shares in the local zone) are considered unsafe.

But, the blocking was only added to XP by a sp (sp2 afaik)

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I think your assumptions are wrong. It's not saving a file from non-NTFS volume that makes it unsafe (wat?! is NTFS magically preventing you from saving viruses or something?). It's saving file from external location that causes Windows to mark file as unsafe, but it requires support in filesystem and only NTFS supports those "insecurity" flags. Anyway, this answer doesn't solve OP's problem. – gronostaj Nov 24 '13 at 9:48
I mean that those "flags" are only available if you save the result on an NTFS system, since it uses forked files. Therefore, on other filesystems, the "safe" information is not preserved, which makes all files on such volume potentially unsafe since the system can not determine if they are local or files with external origin. – Marco van de Voort Nov 29 '13 at 15:26

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