I am unable to open any .pps extention in widows 7 live mail even though I have 2007 power point viewer in the default associated programs. I can only open if I save to desktop first.
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"