Ideas and things to do:

"I didn't know I had a choice" or "What choice did I have?"
Linux supports older hardware, and older PCs usually perform nicely. Linux supports more hardware out of the box than ANY other OS.
This raises environmental issues such as disposal of old hardware.
Free to choose your interface / GUI
Gnome,KDE, Beryl etc.
You don't have to switch to Linux, you can run open software on your Windows or Mac PCs.
Open Office, The GIMP, Firefox etc.
Security right from the early days with >30 years heritage.
Virus, Firewall, Spyware.
Diversity in OS and applications increases computer security as the larger 'gene-pool' resists attacks on specific security holes.
Fed-up with Activation, Reactivation and constant 'Validation' for software you 'own'.
Is there alternative software?
Software I have paid for is usually restricted to run on just one machine.
Even hardware upgrades or recovery from a failure can temporarily 'suspend' the operation of my computer until I confirm that I am not trying to pirate the commercial software.
Modern anti-piracy measures seem to effect genuine customers more than pirates.
Do anti-piracy measures actualy reduce piracy?
Do I have to use Windows?
Free (as in speech) and Free (as in beer) definitions
What is going on around the world to promote Open Software and Open formats?
If I use an OS or applications which are not the same as my friends, or at work, how can I be sure I can open documents from others, or create documents others can read/edit?
What is hardware DRM, and why do people say it is evil?
Does all this really affect me?
Many modern applications 'phone home' via the internet supposedly to check for updates, but how do I know what data is is collected and sent about me?

Tips to help someone adopt OSS or Linux:
1 - Give them a LiveCD like Ubuntu (obviously).
2 - Point out that it is their choice to try or install Linux - and that is what Linux is all about: Choice.
3 - Advise that most of the time Linux is simple, but at times Linux will be difficult. The positive is that YOU will be in complete control of your computer and the data stored on it.

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