Presumably you do mean Outlook and not Outlook Express? If so have a look to see if there is a shortcut to launch it in one of the "Startup" folders - there will be a Startup folder in the individual User's "Documents and Settings" and also in the All Users' "Documents and Settings".
If there is nothing in there, have a look at the Startup tab in "msconfig". Go to Start > Run, type msconfig and click OK.
For Outlook, you are looking for a shortcut to "Outlook.exe"
For Outlook Express, it would be "msimn.exe"