The Outlook Unread Mail count on the log on screen of Windows XP is the combined number of unread items for all accounts or mail applications that expose their unread count. Aside from Outlook, this can also be for instance Outlook Express or Windows Live Messenger.
If you want to disable the feature altogether, then you can do that via the registry;
Key (current user only): HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\UnreadMail
Key (all users): HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\UnreadMail
Value name: MessageExpiryDays
If this value name is missing, the default is assumed which is 3 (days).