Wait till the first time you catch her butt naked running your built in web cam
Actually wait till she sees how cool it is to be able to live Video Chat or create Video
PodCasts and presentations.
Ms Office actually performs better under Mac OSX
Run iTunes remote controlled.
Find a wireless connection anywhere.
Short of necessary OS updates and software updates, you won't be spending
any time fighting Malware and Viruses.
Until you've used MacOSX, it's hard to imagine not being forced to run
all those Windows A/V applications.
You can actually drag MS Internet Explorer to the trash if you so please.
It's YOUR computer.
You can custom reduce your OS install from the default 12.5 GB down to
as low as 4.5 GB for dedicated workstations.
You can secure it and if need be, encrypt your entire system with ease.
Once you set up your system.
You'll have a Primary Administration Account.
Set up this account for Primary Administrative tasks
like installation of software and maintenance.
Set up 2 Secondary user accounts for daily use allowing
as much permission as you desire, but preventing installation or removal
of software.
Unless someone knows your Primary Administrative password, they can't touch your system.
She's going be fighting you for Mac time before you know it, or she'll be gone.
If so, you may have saved yourself from a dismal future.