One thought: Touching the stylus to the canvas is a left click. If you touch the stylus to the canvas and right click, you'll get a dab of paint due to this.
If you're right clicking and dragging, the best way to do it with the stylus is not to contact the surface of the tablet, but to hold the tip of the stylus just above, press down the right click button and move the stylus to drag the canvas.
Otherwise, if that's what you're doing already, check in your Wacom tablet control panel that you are using the default settings. Also, as you're using Vista, ArtRage is likely using Microsoft Ink Services ( Microsoft's tablet input system ) by default for tablet input so the problem could come from an Ink Service setting sending a left click message to ArtRage.
To disable Ink Services in ArtRage, do the following:
1/ Create a file called no_ink.inf ( the filename is the important part )
2/ Place the file in ArtRage's Resource folder. e.g. c:\program files\ambient design\artrage 2\resources\ <-- file goes here
3/ Start up ArtRage and see if the behavior has changed.
Also, see if there's any difference if you enable / disable Precise tablet mode ( Edit --> Preferences --> Use Precise Tablet Mode, then restart ArtRage )