How can I enable autocomplete for the Microsoft account field when signing in to outlook.com?

I have 2 computers running W7 and one running Vista.  On 2 of the three machines, (one W7, one Vista) when I begin entering my username into the Microsoft account field to sign in to Outlook.com, autocomplete presents suggestions for entry in a drop down which is helpful.

On the other W7 machine, no dropdown is presented.  Apparently I've done something to disable the dropdown and am unable to discover how to re enable the feature. (At one time the drop down did work.)

I've found several suggestions to enable the autocomplete for entry of email addresses within Outlook for use when sending email but not for enabling autocomplete during Outlook.com (or live, skydrive etc.) sign in.

Thanks for any assistance offered.