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.
P.S. In case you are wondering, no, it's not possible to bring Outlook Express to Vista.
In XP, I had different family members creating separate mail accounts in Outlook Express. They simply had to open Outlook Express, enter their password, and receive their messages. However, when I purchased a new computer that came installed with Vista Home Premium, we found ourselves using Windows Mail and when we open the Windows Mail program, ALL of the mail from every user shows up. How do I get back to having separate accounts where only the mail for that user shows up in his/her mailbox? In other words, how do I make it so that when I open Windows mail, I'm not automatically getting everyone's mail in my Inbox (I just want the messages addressed to my email address)?