I do not know how to ask this but, how is it possible to share two sides of the computer? My husband and I each have web sites on two separate "sides" of the computer. How is that? If we have another person ( a neighbor to be exact) how do we stop this person from access to emails on one side? How do we keep him from finding out our passwords or usernames? He did that once before at my prompting (because I lost my Yahoo password). How do we keep him from accessing our past internet inquiries? (I.E. our bookmarks.) How do we erase our bookmarks? Or disarm him in advance?

Set up a Guest account that he would use.
Make sure that you log out when you step away from the computer for a while. Make sure that your screensaver requires you to reenter the password.

By setting up user accounts that are password protected. Each user, then has their own private folders, their own bookmarks and other customizations.