nukpaga,
it would be helpful if you listed what your particular computer case was.
you should also take photos of the rust, host them on tinypic or other site and post links here for us to see.
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no, canned air will not make your pc rust
yes, it could be caused by leaving the window open in coastal areas but in high humidity areas only its likely going to be the same with the window shut unless you have a dehumidifier in the room.
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the only thing you can really do once rust sets in is to remove all of your pc components, lightly sand the case and repaint it with some decent paint. you might want to clearcoat it as well.
since you have a humidity problem i would suggest exterior use paint just to be sure.
you can then put everything back in the case and hopefully you will be fine.
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a case doesnt need to be cheap to have issues associated with being built cheaply. even some $130 cases are built out of mild steel and if they are not painted properly can most certainly rust in the right environment.
again, what case? and link photos.