A security update for Windows XP and Windows Vista (included by default
with Windows 7) may block access to downloaded CHM files to protect your
To solve this, run Windows Explorer, right-click on the CHM file, and select
'Properties' from the popup menu. Click on the 'Unblock' button immediately
below the Advanced button on the General page, and then 'Apply'.
Once the CHM file has been unblocked, the Unblock button disappears, and
the content will be viewable.