Outlook 2007 has a very cool preview feature that you can use for Office 2007 formated docs. You simply select the attachment in the email note and without opening in a new window you get to view the contents of the document.
The only problem is that this doesn't work with all file formats, and one that I get alot of are PDF files. So if you too get your share of PDF files and would like to have previews for PDF's added to outlook chech out this post Ryan Gregg. He has made a PDF preview handler that uses the Adobe Reader control to render inside of Outlook.
If you aren't intrested in having Acrobat Reader on your system you can try Foxit.
Microsoft, Outlook 2007, PDF, Preview, Addon, Software, Downloads