I am subscribed to an RSS feed in Outlook that requires a username and password. It's been working fine on my old computer with Outlook 2007. Now I've installed Office 2010 on a new computer and Outlook cannot get the feed. I've looked all through the Properties dialog for that RSS feed folder and don't see any way of updating the credentials. I don't want to delete and re-create it because the main reason I'm pulling this feed into Outlook is I want to archive it. Old posts drop off the feed and I want to keep them.
Try visiting the RSS feed in internet explorer and when asked for username and password, click remember so that it stores the credentials.
This should be enough so that next time you use Outlook, the username/password will be remembered.
... I cannot find a password protected RSS feed for me to test it on, but this is how I expect it should work, if anyone has one that I can test on, I will test it out.