Although it isn't an adventure game, I would have to recommend Starcraft with the Brood War expansion pack.
This RTS can pretty much run on any computer and it has a pretty good storyline. There are thousands of people who continue playing Starcraft: Brood War online (including myself). Picked this game up around 4 years ago and I still continue to play it. I have only started playing competitively around 1 year ago, though.
It is also only $10 for Starcraft and $10 for the expansion now a days, so you really won't be spending much to just try it out.
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