I have just tried to open a file with a .chm extension on a computer running Windows Vista. The file is located on my desktop yet when I tried to open it only the table of contents would display and not the actual pages containing any content. Instead I was presented with a screen that said: 'Navigation to the webpage was cancelled'.
Way back in 2002 Microsoft released a security bulletin regarding what they called a 'vulnerability in HTML Help that Could Allow Remote Code Execution'. The issue is related to help files that many software vendors use in the form of compiled HTML help files that use the .chm file extension.