If you installed security update 896358, changes to the InfoTech protocol prevent downloaded CHM (Compiled Help Module) files from being opened.

The changes that 896358 made to the InfoTech protocol reduce the security vulnerabilities in HTML Help, causing Attachment Manager to treat the downloaded CHM files as un-trusted.

To workaround this behavior, you can clear the Always ask before opening this file box in the Open File-Security Warning dialog and press Open, or you can right-click the CHM file and press Properties and Unblock.

NOTE: See 896054 You cannot use certain Web applications that use the InfoTech protocol after you install Windows Server 2003 Service Pack 1, MS05-026 or MS04-023.