First my credentials: I have an M.Sc. in experimental particle physics and spent six months at CERN collecting and analyzing that data, so this is the voice of experience, not guesswork. I was there in the year 2000, and have since moved into teaching.
She'll want to be running the CERNlib under Linux. You can download that here. CERN is the world's biggest particle physics installation, and they produce PAW (Physics Analysis Workstation) and GEANT (forget the acronym; it was in beta and unavailable in my day) which are the standard software tools for particle physics analysis. CERN also releases their own version of Linux, available here. That's what she wants to be running for her goals, on a computer that has well above average processor power, memory and hard drive performance, but nothing particularly impressive for video and sound.