The cheapest Linux box Dell sells starts at $849, (once you find it), while they sell Windows desktops for as little as $314 (with a free CD-burner/DVD).
And when PJ asked for details, their 'factory installed Linux' consisted of making an appointment for someone to come over to her place and do a Linux installation for her. (for extra cost, of course). For me, this feels like the packages of toilet paper that have a Recycle logo and the notice: "100% recyclable".
On the other hand, she can easily buy a box from Mandrake properly installed for $449.
I also have the luxury of going down to the corner store and getting a no-name box (40GB/CD/256MB/AMD2700, onboard video/audio/ether) for $289CDN (that's about $225US) then adding my choice of OS. No Microsoft tax there.