Do you have samples of what you are taking?
Firstly, you'd need to look for the light. Even indoors, lighting may not always be even. People standing near light sources will be more easily exposed for than those standing in a dark corner.
Next, you probably need 1/125 to 1/180 to freeze motion for people walking about.
Expose for the lit areas only, trying to see how that gives a setting that balances detail captured in the exposure to shutter speed and acceptable ISO. I'd say that 3200 should still be fine for Kx. Very often if the rest of the ambient is dark, the camera is fooled to exposure for these areas, resulting in a slower shutter speed. These ambient areas don't hold any interest anyway.
Shoot in RAW to get the maximum headroom to do highlight/shadow recovery.
Paint.net is fine if you know how to use if with the correct plugins.
Here are 2 suggested recovery processes :
Straightforward
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1. Go get the Tanel Plug-in pack (
http://forums.getpaint.net/index.php?showtopic=11619 )
2. Use the shadow and highlight recovery
Complicated
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1. Go get Pyrochild's plug-in "curves+" (
http://www.softpedia.com/get/Multimedia/Graphic/Graphic-Plugins/CurvesPlus.shtml )
2. Do 2 exposures from your 1 image. One exposing for shadows, one exposed normally.
3. Use Curves+ : Advanced "Luminosity > Alpha" and for the pict with shadow detail
4. Adjust the curve so that only the shadow detail remains.
5. Merge down to the normal exposed pict.
You may be able to find a good tutorial for the 2nd method online.