It looks like something is interacting with the tablet data received by ArtRage. My first guess would be that it relates to the Ink Services ( Microsoft's tablet input system for Vista ) configuration on your system.
To test this, disable Ink Service use in ArtRage by placing a file called 'no_ink.inf' in ArtRage's resource folder
e.g. c:\program files(x86)\ambient design\artrage 2\resources\ <-- file goes here
It doesn't matter what type of file you use for this, just the final name of the file is important ( no_ink.inf, make sure the extension is correct ). Then start up ArtRage and check to see if the behavior has changed.
If that doesn't help, feel free to email me directly at email@example.com, and I can troubleshoot it with you in more depth.