Well, there's no real way of 'winning' this from someone who sold his soul to M$, but you may give him some food for thought by showing him this:
http://en.wikipedia.org/wiki/Supercompu ... ng_systems
Or, maybe more telling, the stats on which that graph is based: http://www.top500.org/stats/list/36/os
In short, of the top 500, only 5 run M$, 30 more are neither Linux nor M$ but the rest is Linux. That's 465 to 5...
Further: you can tell him a modern Linux distro will happily run on hardware M$ has given up on years ago, like the 386 and 486. With graphics, provided there's enough memory (128MB, yes that's a fraction of what Win7 needs as minimum (2 GB, IIRC)) and a suitable graphics card. And that out of the box (more like: straight from the install CD) Linux supports more hardware the any Win-OS, only that there are more hardware vendors providing drivers for Win-OS then for Linux.
To rub salt in the wound, tell him that IE is no longer the dominant browser in Europe linky