http://interruptions.net/literature/Speier-ICIS97-p21-speier.pdf

This is an academic paper, so you probably won’t want to read the entire thing, but the results of the research have interesting implications for "organizational electronic mail systems that have been implemented to enhance productivity."  I’ve always said that workers should disable the global alerts in Outlook that notify them every time a new message is received.  Now this study confirms how bad desktop alerts are on concentration and productivity!  The findings from this research suggest that instant notification features be disabled in order to avoid exacerbating the number of interruptions knowledge workers receive.

Go to Tools, Options, Preferences tab, Email options, advanced email options, and uncheck the four boxes under when new items arrive in my in-box.