Outlook Express
C:\Documents and Settings\[username]\Local Settings\Application Data\Identities\{EF95...0DD}\Microsoft\Outlook Express

Windows Address Book
C:\Documents and Settings\[username]\Application Data\Microsoft\Address Book

Microsoft Outlook
C:\Documents and Settings\[username]\Application Data\Microsoft\Outlook
-or-
C:\Documents and Settings\[username]\Local Settings\Application Data\Microsoft\Outlook