There are a combination of issues that can be involved but the primary issue that causes most if not all of the problems experienced with exchanging email and attachments is using HTML for message composition which is an issue that Microsoft wrought upon the internet. If HTML was banned for use with message composition and everyone was required to use Plain Text, there would be few if any problems.
1) How to explain to a Windows user how they can modify their Outlook mail preferences to either view our mail attachment(s) inline (in the context of an e-mail), OR as non-viewable, but identifiable file attachment depending on their preference (and assuming this is at all possible).
I don't believe Outlook includes such a preference setting.
Mail displays all photo/image and single page PDF attachments inline or viewed in place within the body of the message by default for received and sent messages which cannot be turned off. Likewise, Outlook includes no such setting but I believe Outlook includes a preference setting to convert all message text for received messages to Plain Text.
RTF with Tiger Mail is really HTML but certain things must be done or occur to implement/initiate HTML with a sent message such as selecting or changing the font, font size and/or color via the Font panel when composing a message, including a signature which was composed with RTF or HTML or copying/pasting HTML formatted text from a website to a message.
When adding photo/image or single page PDF attachments to a Mall.app message using RTF for message formatting with HTML implemented, Outlook (and depending on the Outlook version used by the recipient) may receive such attachments as embedded (such attachments will appear in the message body) and not as true attachments to the message which cannot be saved outside of the message even though the Mail.app sends such attachments as true attachments to the message and will be received as such when the same message is sent to a Mail.app user or to a recipient that uses a different email client other than Outlook or depending on the Outlook version used.
The Mail.app and no email client is completely bug free but Outlook has its share of bugs for such an old/mature email client.
But again, the underlying problem is with HTML formatting included with a sent message. Unless HTML is implemented when using RTF for message composition with Mail, the message text will be sent and received as Plain Text but using Plain Text for message composition with Mail and with any email client is best all the way around.
2) How to explain to an Outlook user specifically how they can save a file attachment they are currently viewing inline in the context of an e-mail to their hard drive (sure ...easy on a Mac ...just as easy in Windows??)
If such a message attachment was not sent as a true attachment to the message or received as such (embedded in the message body), I'm not sure this is possible.
We have no control over the email client used by each recipient included with a message we send but we can control the formatting we use for sent messages and using Plain Text avoids most problems.
http://www.birdhouse.org/etc/evilmail.html
http://www.georgedillon.com/web/htmlemail_isevil.shtml
http://homepage.mac.com/thgewecke/woutlook.html
http://www.ncmug.org/tips/mail_attachments.html
I know very little about Vista or the current Outlook version included with Vista or if older Outlook versions are compatible with Vista but Vista by itself probably has no effect in this regard.