If you use mutliple user login accounts in Windows (like when you first boot, it'd ask you for a password), it'll seperate everyone's Outlook accounts so no one can see anyone else's, I believe. If you don't have seperate login accounts, you can set up a Rule in Outlook maybe. Create a new folder called "Inbox2" or whatever, then go to Tools, Rules Wizard... Create a new rule that says whenever mail "Has (insert email here) in the To box", "Move it to (Inbox2, or whatever)". Then it'll seperate your mail after it downloads.

The bad thing is, you'll still be able to read the mail of the person you're sharing Outlook with, and they'll still be able to read yours too. You'll also share a Sent Items folder and a Deleted Items folder unless you create two more folders and two more rules to seperate it there too. Maybe there's some other way to do it, but that's the way I can think of.