Security Key Needed for a newly installed MS Outlook

I have set up Outlook 2003 in my new Windows 7 computer. Everything is working well and everything is set up correctly.
However, all of a sudden I got a message stating that the security key was is needed.
I checked on my Gmail account since it is connected with my Outlook account and it is still working.
Why the error does occur?
I have security key which I already entered during the process of setting it up. How can I fix this?
