If you are plugged right into the sound card, with no microphones hooked up, the fan noise etc. *should* not be the problem....
The monitor on your computer may be interfereing with the keyboard somewhat. If the sound vanishes when you move the keyboard away from the monitor, then that's the problem. (It's possible that the fan could do this also, even without mics involved; electric motors always emit some radio waves.)
The hissing may also be the noise floor of the keyboard (if it's a cheaper model), the "natural" noise it makes when "on" but not being played. Maybe the level of the keyboard is too low (volume boost on low signal will raise all background noise also)--or too high (the keyboard's noise floor comes through).
Unfortunately, hiss can come from myraid sources (and often from more than one source at a time!). You might have to futz around a bit to find where it's coming from.
How loud is this hiss? Many "pro" recordings have detectable hiss on them-- it's just REALLY quiet.