My g/f was already a console gamer (played Halo 2 and Halo 3 online, Gears of War 1 and 2), so my job was easier than yours is. However, I got her into PC games by using Peggle, then Plants Vs. Zombies, then COD - World at War campaign and the Nazi Zombies mode, then the Serious Sams, and L4D, UT3. And finally Borderlands. (OMG Borderlands is SO MUCH FUN, my g/f and I play 4-5 hours a night, but that's another story).
Coop games are essential for introducing g/f's to PC games. If you're not going to play a game with her, she'll become confused and lose interest. Play lots of coop games until she feels comfortable with the mouse and keyboard setup. It's completely foreign for a non-gamer to get used to the mouse+keyboard setup.
Give it some time and BE PATIENT. If you get frustrated with her, it'll make her embarrassed and she'll never want to play again because she thinks she's stupid or doesn't want to be embarrassed.
Help her when she asks for help, but let her figure out what to do on her own. You're not helping her by constantly pointing things out to her. She'll start to depend more on you showing her where to go next, instead of learning how to figure that out on her own.
Better learn how to forward ports and how to host your own games as well, that'll be crucial to coop play in some games.
Remember, COOP is king when introducing a girl to games. Thankfully there are lots of good coop games out there, so good luck!
PS. There are lots of fun Coop games for consoles too... Halo 2 and 3, plus the new ODST is great coop fun on console. Gears of War 2 has good coop as well (GoW1 was just frustrating, so feel free to ignore that one).
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