The viewer may be seeing the Live mail attachment as insecure. One solution would be to use an older version, before the security feature was introduced. Here is a download for the 2003 version, for example: Free Microsoft PowerPoint Viewer 2003 Download
But first, try right clicking the attachment and select "Open with". Browse to your Viewer - it will be in the Office folder and select.
Make sure you also select the small square option, bottom right, "Always use this...etc"