I just answered a similar question here:
http://forums.omnigroup.com/showthread.php?t=6441
A few other thoughts for your specific situation:
1) If you're only using one computer, you only need a single license, not a family license. Though since you also mention your secretary, I doubt this is the case.
2) As steve says, your best route for multiple users is to have multiple user accounts. Each user's OmniFocus file will be stored in their own user account by default.
3) "And my secretary has access to my task list and will occassionally modify it." How do you plan to set this up? Unless she's literally just sitting down at your computer to make changes, you could cause some nasty data corruption by editing the same Focus database from two computers at the same time, or end up with two separate, unmerge-able databases. If she's just modifying the iCal calendars you're syncing to, then you could choose which calendars get synced and not sync your personal stuff.