If you edit a standard TwistedBrush ArtSet on Windows Vista or Windows 7 the ArtSet file is copied to the Virtual Store which is a Windows security measure. Now you have the edited ArtSet in the Virtual Store and the original ArtSet in the TwistedBrush\ArtSet folder. When an update installation occurs the original Artset is updated however Windows will only point to the edited ArtSet in the Virtual Store. To remedy this situation you can manually delete the ArtSet file in question from the following location.

\desktop\username\AppData\Local\VirtualStore\Program Files\Pixarra\TwistedBrush\ArtSet

Note if you are running the 64 bit version of Windows the location will be
\desktop\username\AppData\Local\VirtualStore\Program Files (x86)\Pixarra\TwistedBrush\ArtSet