Try this http://www.nirsoft.net/utils/pst_password.html
It supports Outlook 2007, and I'd suggest you make a backup of the file before using any tools to try to obtain the password... last thing you want is the file to become corrupted.
Hmmm - contains a file called mailpv.exe which McAfee immediatley deleted. A few Google searches later reveals this particular .exe to be a little unfriendly!
Original source, Israel.
I've used plenty of things from Nirsoft before and they've been absolutely fine - they make some very handy tools.