I can't begin to emphasize enough how important the email indexing tools from Google, MSN & Yahoo are. Somehow the almighty Microsoft corporation forgot that it's hard for humans to find data on their hard drives because the Windows search functionality is painfully slow. And the Outlook search isn't much better (it's OK if you have very little email data to sort through).
But even if you do think Outlook's search is OK for you, download & install the Google & MSN indexing search tools anyway. Because at some point there will be thatone email you have to find, and without these tools you're basically groping around in the dark without a flashlight. There have been times when my associates have asked me to find an email I sent them that they've spent hours looking for, and usually I can find it in 20 seconds or less. This tool is also GREAT when you have a dispute with someone and you need to pull out those emails from 1, 2 or 3 years ago to prove you're right.
I hear the Yahoo tool is also good, but these two pretty much cover my bases.
Speaking of being able to grab data from long, long ago, you should also download Trillian (www.Trillian.cc), which is an awesome Instant Messaging tool that lets you connect to AIM, Yahoo, MSN, Google Talk & others right from one application. AND it automatically logs your IM conversations, meaning you have a history of the IM chats you've been having - again a very valuable tool for proving a point to someone who might've forgotten a certain conversation.