To link to a UNC path from an HTML document, use file:///// (yes, that’s five slashes).
file://///server/path/to/file.txt
Note that this is most useful in IE and Outlook/Word. It won’t work in Chrome or Firefox, intentionally – the link will fail silently. Some words from the Mozilla team:
For security purposes, Mozilla
applications block links to local
files (and directories) from remote
files.
And less directly, from Google:
Firefox and Chrome doesn’t open “file://” links from pages that originated from outside the local machine. This is a design decision made by those browsers to improve security.
The Mozilla article includes a set of client settings you can use to override this behavior in Firefox, and there are extensions for both browsers to override this restriction.