Nothing kills my productivity at work more than interruptions, from both people and computer based interruptions.  Sometimes those interruptions are warranted but frequently they're not.  One of the main killers is Outlook - it keeps popping up those cool e-mail notifications.  I think they're neat but I tend to pay too much attention to them.  43Folders.com has a set of 5 e-mail productivity tips that I'm going to start following tomorrow.

http://www.43folders.com/2005/02/five_fast_email.html