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Nice!
I'm starting to think the patterns you see (as in the dark frame you posted before) are caused by a repetitive/modulated type of noise (hum) - it's clearly not random noise that you can 'stack out'.
Depending on how still the image is between multiple frames (it shouldn't drift, but my educated guess from looking at the noise is that your polar alignment was very very good anyway), you may want to try to add an effect in Virtual Dub that averages a few frames together, such as a motion blur. Then maybe take the result and stack the resulting individual frames. This should hopefully blur the hum (but will keep Saturn still) so that the stacking algorithm won't detect too large peaks caused by the modulated noise.
The reason why this (might) work is that, if the noise isn't random, on average some pixels 'inherit' the noise more often than others, causing a pattern to emerge.
I'm thinking another way you could somewhat mitigate the problem is by not perfectly polar aligning your mount (introducing dither). This will cause Saturn to appear at different spots and will make Registax align the image accordingly. This means that the modulated noise moves (relative to Saturn) and will occur at a different spot every frame. This will help the stacker decide it's noise and not a real signal. Even better would be to move around Saturn around slightly in random directions every few frames (during capture - not in post processing!).
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