Under XP, Outlook Express let each Windows user account have multiple identities, supporting multiple user mail accounts. That feature was done away with under Vista, so in order to keep them separate each person must have his/her own Windows user account, which can be created by going Start->Control Panel->User Accounts. Then, if you're importing mail from a backup, you'll need to import it all, delete those that don't apply to the suer, and then repeat for each additional Windows user account. In cases such as yours it's far beyond inconvenient, but it's the only way under the new system.

Sorry.
John


P.S. In case you are wondering, no, it's not possible to bring Outlook Express to Vista. Sad