Just another point, I would recommend educating her about open source software in general. Tell her that alternatives to Microsoft software like FireFox and OpenOffice do exist. Let her make her own mind up, you never know she might like OpenOffice and if she doesn't then fair enough but don't bother arguing with her about it. I'm best of friends with my sister and I wouldn't let Microsoft fuck up a good friendship, Bill Gates just isn't worth it.
When we got our new computer (my father, bother and I payed an equal share toward it as we're the main users) it came with MS works, I installed OpenOffice, and managed to persuade both my father an brother to use it because I demenonstrated to them how much better it is. I also tried to get them to use FireFox but my dad prefurs IE for some reason but I managed to convert my brother to Opera.
I then set up everyones user accounts as limited and made an administrator account and put the password on display, often people complained because some programs produced the odd error, but I explained to them why it was best to use limited accounts and how to use the run as administrator menu. Now I rarely have any complaints the system runs well and I accept people use different software.
