I recently ran into an issue viewing a chm files in vista. Looks like chm files from untrusted sources are blocked by default. There is an easy fix though.

• Right click on the offending chm file
• Click properties
• Click unblock

The help file should open correctly now. If it doesn’t make sure you aren’t opening it from a network location. Chm files do not work on network shares. Just copy it local and follow the steps above.