Outlook XP has the same nice AutoComplete feature that IE does.  However, Outlook does not have an intuitive way of deleting incorrect items that AutoComplete remembers.  For instance, lets say I want to send a message to joe@jos.com but instead I type joe@jop.com.  Now when I am typing a message to joe@jos.com the AutoComplete feature will give me the option of using joe@jop.com before it gives me the correct one.  There are two ways (that I know of) to solve this without turning off AutoComplete:

1) When you are typing a message and see the wrong name appear, select it (using the up and down arrows) and hit delete.

2) If you've really messed things up you can clear all the information in Outlook's AutoComplete database by deleting the .nk2 files created by Outlook.  This is most likely located in C:\Documents And Settings\<username>\Application Data\Microsoft\Outlook        NOTE: you will have to have hidden folders viewable to see the “Application Data” folder.