Hi Jim,
One of our PCs is a dual core Pentium on XP with a Nvidia 7950GT graphics card with driver "02/05/2008 version 6.14.11.7516 " running dual monitor. We had the same issue using the paint effects brush leaving an annoying trail behind it as the user moved the mouse
The good news for us was its an easy fix
Our solution was simply to open up the Nvidia control panel and under "Manage 3D settings" add the maya.exe to the list of programs in the drop down box, then set Antialiasing - Setting to None
Job done, opened up maya and paint effects tool brush worked exactly as intended without any annoying trail
Hopefully this will work for you also.