According to the article written by Lawrence Latif, N.A.S.A. will replace the Windows XP with Linux :
THE INTERNATIONAL SPACE STATION (ISS) will remove Microsoft Windows from its onboard laptops and run Linux exclusively.
The main reason can be :
...NASA has been using Linux on its missions for some time, including on its Mars Curosity rover, suggesting that the delay in shifting over to Linux on ISS was more to do with porting legacy applications from Windows.
Read more about this here.