I have a problem opening my *.chm notes recently. I quickly google for solutions and fortunately i got lucky. The only problem was, i tried almost all the suggestions from registering an regsvr32 hhctrl.ocx to registry editing not knowing that the solution is very simple. To save some time, try to follow the steps below.
Solution 1: Right-click on the CHM file on windows explorer, and select Properties from the popup menu. Click on the Unblock button immediately below the Advanced button on the General page. Click Apply. If you can’t see the Unblock button it only means you have to go to Solution 2.
Solution 2: Make sure that there are no special characters (#,%) in the directory path where the .chm files are located. Also avoid using ‘?’, ‘&’, and ‘+’ characters in directory names.
Solution 3: Run “regsvr32 hhctrl.ocx” command from the command line to register the library in the system.This ocx file might be corrupted or unregistered.
To be able to run this on vista/windows 7, User Account Control setting must be disabled.
The most convenient way is simply search for command prompt in start menu then right click, select Run As Administrator and then paste regsvr32 hhctrl.ocx.
Your last resort,
Solution 4: Modify the ItssRestrictions registry entry in registry editor. You can find the detailed information here.