For correct sampling of a deep sky image the pixel size should be one half to one third the size of a typical star's disk enlarged by seeing. For most parts of the country this will be about 2 to 3 seconds of arc. So if your seeing is say 3" of arc you want a pixel size no smaller than 1 second or larger than 1.5 seconds for best sampling and ability to get the full resolution your skies allow. This is for deep sky imaging. It doesn't apply to planetary work. There a typical web cam running at f/25 to f/30 is what you want. Oversampling images (pixel too small) just increases exposure time without any gain in image quality so is to be avoided if at all possible. Undersampling (too big of a pixel) can result in odd looking stars (square). But it does reduce exposure time and is often done intentionally for wide field photography. There are ways to deal with the star shape as well so this isn't necessarily bad. But you do give up resolution to get this reduced exposure time and wider FOV. There's no free lunch here either.

The formula is:
Sampling in arcseconds = (206.265 / (focal length in mm) )* (pixel size in microns)

Your camera has non square pixels so the images will have to be resized to square them up or the image will be distorted. Orion's software may do this automatically, I don't know. For the formula use the larger dimension 8.6 microns. With a f/6.3 reducer/flattener you'd have a 1260mm focal length. That would result in about a 1.4" pixel scale for a bit over 17 minutes FOV on the long axis. Short one would be about 13 minutes. Pretty small but good for planetary nebula and many galaxies. Without the flattener/reducer it would be about 11'x8'. Smaller than I'd want to work with. Just finding the object would be a pain beyond reason and very limiting. In any case the pixel scale is reasonable. Field of view a bit limiting however. You might want to look into an even stronger reducer but they can be really touchy as to mounting distance between camera and reducer that if not correct can create problems you don't need.

Wodaski has calculator on the net you can download at:
http://www.newastro.com/book_new/camera_app.php

I'm not sure how it handles non square pixels.

You mention Registar. That is very different than Registax. Though I've not heard of a problem running either. You may have a faulty download. Registar is for aligning CCD images for stacking or mosaics and has nothing to do with web cams nor is it close to free though you can download a version that can't save images for free. Registax is probably the best planetary web cam processing program. I've not heard of any issues running it under Vista but haven't tried it. It works fine under W2K and XP my two operating systems. K3CCDtools is another but it is pay for the best version. I have heard of that not running under Vista yet others had no problem.

Rick