The Portal series is a great place to start, as both are known for being 90%+ tutorial for the player and entertaining the whole way through. Not too difficult, and puzzle games aren't too demanding in terms of reaction times and that; controls are quite simple as well, and introduced at a rate that's perfect for gaming newbies. To top it off, the games aren't too violent nor offensive in any way (GlaDOS' scathing remarks aside)... though the humour can be a bit dark at times. Portal 2 comes with a co-op mode as well, though it's recommended that both parties got into it dark (haven't played the co-op mode before); that way you can solve the problems together, not just have one guide the other through it.
For other PC suggestions? Maybe the two recent re-releases for the Monkey Island franchises (for the first two games of said franchise, called "Special Editions"). Point-and-click adventures are a bit dated as a genre at this point, but those are whole-heartedly entertaining and hilarious.
And so far, everything is available on Steam.
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Now for the mildly tricky part, platformers. While it was requested to keep things on the PC, platformers are a bit rare on it; Nintendo has some of the best ones on the Wii, if you're able to get her on it. The reason you want to look at platformers is because they're more or less pure gaming fun; and as a result, a great way to get someone into gaming (by showing them what it's all about). While the genre definitely has a "for kids" look to it, anyone can have fun playing them; they're just that damn good.
Super Mario Galaxy and it's sequel are probably the best to go with, both of which received near-unanimous praise from both critics and players. Again simple controls are at work here, but they also have motion controls; while that might seem off-putting at first, the game's design knows they're new to everyone and eases players into them. Someone who's completely new to gaming might actually have an advantage here, as they don't have to fight the muscle memory developped from years (or a couple decades) worth of playing games with a standard controller.