Completely random problem with one of the machines here: the mouse pointer was offset about 30 pixels or so to the right, so that the hotspot was about 30 pixels left, and the mouse wouldn't move all the way to the left-hand-side of the screen. (Similarly, it would scroll that far off the right of the screen.)
It turns out that it's a bug in the XFree86 Trident driver. If you've got Driver "trident" in the video card's Section "Device" in your XF86Config-4 file, then try adding Option "SWCursor" to that section, and this should fix the problem.