Although this doesn't probably apply in your case, keep in mind that any business or industry specific software is almost certainly going to be Windows based. My wife works for an airline, and the software they use to check schedules is only available on Windows, for example.

Also, if you get a new Windows based system, be aware that many are coming with Vista-64. It is a better system, but it is does not have nearly as much 3rd party device support (scanners, cameras, etc). Vista 32 is a better option at least for now for less experienced users.