The HomeBase Desktop is a desktop interface which is based on Mozilla browser and runs on top of Linux. I mention it here because it shows that there are a lot of folks out there working on trying to make a viable and attractive alternative to Microsoft Windows and more importantly they’re getting better at it.
For someone who only needs the most basic of functionality (WP, browser, calendar etc) then this could be a real alternative. Of course the real problem comes when you want to use the PC for a bit more like digital photography, video editing or even running Blog! :-)