Disappearing/flickering mouse cursor

Hi - I'm finding that on some machines the mouse cursor disappears
when the mouse is not being moved. As soon as the mouse is moved, the
cursor reappears, though it flickers while the mouse is moving. Then
when the mouse stops moving it disappears again. I have used
ShowCursor to make sure it's on (if I don't use ShowCursor then, as
expected, the cursor does not appear at all). This only happens on
some machines - it happened yesterday on a ThinkPad T60 laptop running
Windows XP. The code for the program we're running is huge and I have
no idea what's causing it so I can't send you it, but I can say that
the PTB functions we're using are screen(OpenWindow,
OpenOffScreenWindow, TextBounds, FillRect, DrawText, CopyWindow,
DrawDots, and Flip), ShowCursor, HideCursor, GetMouse, KbCheck, and
GetSecs. There are no calls to HideCursor getting in the way -
HideCursor is only called later in the code. I also have the text
renderer set to high quality renderer using Screen('Preference',
'TextRenderer', 1). Any ideas?