There is a file within Outlook which auto completes email addresses when you start typing them;

The location of the file which holds the email addresses is an NK2 file and is different on each version of Windows & Outlook. The Nk2 file name will be the same as your profile name in Outlook;

 

Outlook 2003 with...

• Windows 2000 - C:\Documents and Settings\%username%\Application Data\Microsoft\Outlook
• Windows XP - C:\Documents and Settings\%username%\Application Data\Microsoft\Outlook
• Windows 2003 - C:\Documents and Settings\%username%\Application Data\Microsoft\Outlook

Outlook 2003/2007 with...

• Windows Vista - C:\Users\%username%\AppData\Roaming\Microsoft\Outlook\
• Windows 7 - C:\Users\%username%\AppData\Roaming\Microsoft\Outlook\
• Windows Server 2008 - C:\Users\%username%\AppData\Roaming\Microsoft\Outlook\

Outlook 2010 with...

• Windows XP - C:\Documents and Settings\%username%\Local Settings\Application Data\Microsoft\Outlook\RoamCache\
• Windows Vista - C:\Users\%username%\AppData\Local\Microsoft\Outlook\RoamCache\
• Windows 7 - C:\Users\%username%\AppData\Local\Microsoft\Outlook\RoamCache\
• Windows Server 2008 - C:\Users\%username%\AppData\Local\Microsoft\Outlook\RoamCache\
• (Slightly different under Outlook 2010 its a "DAT" file looking like "Stream_Autocomplete_0_877840B38D2ECC48AB9D00728C7CAB63.dat"