It will no longer be possible to access .chm files over the network following a Windows XP security update. Although the help file will be launched, the content will not be displayed correctly. If you copy the CHM file from the server to the local computer, the file will work properly.
Following the discovery of a security flaw in these .chm files, Microsoft has made it impossible to run these files from the network.
Further information is available at support.microsoft.com/kb/896358