Even though the ARR website lists Vista as the official minimum requirements for the PC version, I can accurately say the game will run under XP SP3 (the Alpha and Beta versions were designed for it as a minimum) and I still play ARR in XP). However, as should be obvious, SE has dropped all technical support for the game running under XP, so if something goes wrong that is caused by your operating system, they won't be able to help you I'm afraid.

But really, if your girlfriend's computer is reasonably up to date it should be able to run ARR with no problems (especially if she's a 'PC master race' gamer like you said. ).