Outlook 2007: forcing an exact search on a character string?

Hello,

I want to select messages where the title (and only the title) contains a
specific numerical reference, for instance "123456789".

When I enter this "123456789" string in the "subject" field of the "All Mail
Items" advanced search bar (or use "subject:123456789" in the simple search
field), the application also returns the messages where the title contains
just the beginning of the string, for example, titles including "123456000"
or "123456123"...

How can I force the search function to return _exact_ matches?

Outlook is running under Vista Ultimate. I would add that I used to run
Outlook 2007 under XP Pro and the search worked fine at the moment... It
doesn't anymore with Vista.

Would that have to do with the search indexing of Vista vs. the one added as
an option in XP?

Advice, someone? Thank you!

Renaud