I'd start with worrying about windows. The drivers mainly just make the Guns appear as a HID-input/mouse. So you should be able to use your gun to track around your windows desktop. Start here - if its dodgy here, it will be dodgy everywhere.
Note you will have to expect some "jitter" - this is where even if you mount the gun on a tripod to hold it steady/lie it down stationary, you will see the cursor on screen move around slightly. This is normally just a few pixels and not significant when playing (after calibrating).
NOTE: the gun CAN NOT track on dark areas of your desktop/screen. If you have a picture background or something, this CAN LOOK like its jumping all over the place, when really its only tracking on the light bits. Beef up the brightness of your screen or remove the pic. (Note upping brightness of the screen is something worth trying anyway).
Once auto calibrating, I believe both available drivers allow some manual tweaking. General consensus is that while autocalibration is pretty good, you can probably improve on it slightly.
If your pointer is moving around a lot (more than "jitter") then it could be a lot of things. I've not used a scan converter, but I can't imagine its helping at all - in fact its almost definately not. The best way to tell for sure is to simply test without it. (Seeing as you are using a scan converter, I imagine this would require you to use tv-out from your pc). See if its much better.
Once its happy in windows, then I believe some gun games have there own configuration. Turn on the setup dip switch in games, or delete the "nvram" file for that game - when next running it should then run through the calibration screens (This depends on the game).
Hope that helps.