It isn't accessible in the registry. The OE logon UID and PW are the same as for SBC Global. If you need to change OE's, you need to change it at the SBC Global logon site first. So, you didn't forget it. What exactly is happening? Anything here help?
Outlook Express keeps prompting for your password
http://www.dougknox.com/xp/tips/xp_oe_passwords.htm
Save password setting not retained in Outlook or Outlook Express
http://support.microsoft.com/kb/290684
Save Password Check Box Is Unavailable
http://support.microsoft.com/?kbid=137361