If you find Outlook getting cluttered with windows when you're replying to mails, there's an easy way to clean things up. This works in both Outlook 2007 and Outlook 2010.
Just go to File, Options and select the Mail tab. (For Office 2007, use the Office button.) Now scroll down to Replies and forwards and check the box to Close original message window when replying or forwarding:
While you're here, you can change how replies and forwards are handled and even add a default message to precede any comments you might add. This is a good place to enter a generic note that summarizes your typical response scenario, such as "Take a look at this and let me know what you think." You can always remove or customize the message before sending.