I would go with what retrocity said, mixed with the last of what cryptojoe said.... that's why I pointed at the mouse... I don't use an optical mouse for the simple reason that they are "flaky"... you can be pointing at something, and suddenly have the cursor somewhere else on your screen.... for no known reason. the styli come in two types... optical..... registers the screen raster, and tablet, which is a positional feedback from the tablet itself. which type are you using? One thing you might look at, after testing on a different system, would be the Report Rate.... too slow, or too fast can have a similar effect. Lou. P.S. I find it really hard to believe that your HDD would be the culprit.

"..... and that was when things got interestiing."