If you answer e-mail questions "inline" – that is, inside of the sender’s original quoted message – you should use Outlook comments. This marks your insertions with your name so that the recipient(s) can quickly spot your notes and you don’t have to type ">" in front of everything you add. (You should still make a note that your "answers are below" at the top of the mail!)
In Outlook 2007, go to the main application window (not your message), click the Tools menu, and select Options. Now click the E-mail Options button, check the box that says Mark my comments with, and add your name in the box below it. Then click OK for all open dialog boxes and return to your mail.
In Outlook 2010, you don’t even need to leave the message. Just click to the File tab at far left, click Options, and then click to the Mail section. Scroll down to Replies and forwards. Now check the box that says Preface comments with, add your name, and click OK to return to your message.
If you ever need to turn it off (say, if Outlook is adding comments when you reply to someone and delete the body contents to start a new mail topic), simply reverse the steps. Just don’t forget to turn it back on when you’re done!