What is the intended usage for your audience? Are they considering Linux for personal use, a small business office, medium-sized school, large factory, volunteer organization...?
In general, if your clients are interested in using their computers in a multi-user environment, stress how Linux has been designed from the very beginning for that task. Emphasize how a user is limited to only screwing up his own data -- how he is prevented from destroying anyone else's and how he can't "take down" the system itself.
In a similar vein, provide some anecdotes about companies and organizations whose employees (or staff) have installed unlicensed software and this has resulted in audits, threats of litigation, and oftentimes the "settlement" that they upgrade their entire operation to Microsoft's (or "Company X's") latest software (the case of Ernie Ball
is a fine example; and might be worth providing printouts of that interview to your audience). This topic might be a little ham-fisted, depending upon your audience, so you might wish to put things a little more delicately than I have here.
If your clients are involved in "production" at all, point out the multiple providers of Linux and remind them how undesirable it is to have their company's livelihood dependent on a sole-source supply (I wrote a longer description of this on the old VL forums
a while back).
Depending on the target market, I would promote the networking potential of Linux. Enable your home computer to permit remote X logins from the Internet (with appropriate security) and during your presentation, open a display logged into your home machine and demonstrate how you can switch between the two using CTL+ALT+Fn. Be sure to emphasize how secure the implementation is: accounts have to be authorized, all data transceiving is encrypted, and even limit the login to a single port.
Along the same lines, give one (or more) of your audience members a LIVE bootdisk configured to be a thin-client and let them log into your "display model" and demonstrate how they might turn some of their "obsolete" Pentiums into graphics terminals at minimal cost.
Above all, you need to cater your presentation to your audience; much of what I suggested might not apply to your situation and could even serve to put off your audience. You will also wish to indicate that some of the things I suggested would require expert assistance to set up.
EDIT: You might also consider installing Qemu or VirtualBox on your machine and installing WinXP as a virtual machine. It might be impressive to Windows user to see their entire system being run as an application under Linux.