Windows XP's log on screen displays the number of unread messages for all email accounts checked with Outlook, Outlook Express, and Messenger or MSN Client (Hotmail/MSN) from your user logon within the last 3 days. You can disable this feature or change the number of days it keeps track of unread messages by editing the registry.

Browse to
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\UnreadMail and change the value of MessageExpiryDays or add it as a new DWORD if it does not exist. Use the default value of 0 to disable it or enter any other number to change the number of days.

For more information, see
http://www.outlook-tips.net/howto/winxp_unread.htm