For some time now I have been unable to read downloaded .chm files. Instead of the help contents I got an error page.

The address is not valid

Searching for this on google turned up a KB article that says this is because of a security measure introduced in security update 896358.

To view a downloaded .chm file do the following.

1. Right-click the CHM file, and then click Properties.
2. Click Unblock.
3. Double-click the .chm file to open the file.