Due to Microsoft's hammerlock on OEMs, almost none. Some mom-and-pop computer shops will sell you a PC with Linux on it. Wal-mart has a PC preloaded with Mandriva.
I've heard that you can get a Lenovo laptop pre-loaded and Emperor Linux sells laptops with Linux.
Best bet for your brother is to install something like VMWare and run Linux inside of it.
I'll probably just help him install a dual boot/dual drive with Linux and whatever version of windows his new PC will have.
As always, thanks for the help. I don't always agree with the people on these threads, but you people on the ping list are very knowledgeable!!
You guys even convinced me to give Linux a try ... and I'm a Micro$oft Clone!! I've been using Mandrake, off and on, for over a year. I recommend to all people I know in IT jobs to try both and decide what works best. I like the variety of using both MS and Linux.