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 

system's security. 



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.