I absolutely LOVE Windows Desktop Search (integrated into Vista, add-on for XP).
Some of my favorite queries are:
kind:email from:"firstname lastname" - search for all emails from a certain person
kind:email sent:yesterday keyword1 keyword2... - search for an email sent yesterday about keyword1, keyword2, ....
and a recent favorite:
All of these properties along with the search syntax are included in the "Rules for advanced searches" page of Windows Desktop Search's help.