Microsoft Compiled HTML Help,CHM file is blocked in windows 7 by default, if you want to read chm files format
If you are running MS Windows 7 and you have some books in .CHM format – these files are blocked by default. They will open, but you won’t be able to read any of the pages as the content is completely blocked. This is working same way as in MS Windows Vista.
Right-click on your .CHM file
Click on the General tab
Go down and click on UNBLOCK, APPLY and OK
Now you can re-open the document and you will be able to access all the content