A square? Hmm...
Usually this helps: try lowering your hardware acceleration one step.
Right click desktop>Properties>Settings>Advanced>Troubleshoot>and you'll see the hardware acc slider.
Move it one step to the left.
Or try a different cursor...
Also, if you change pointers a lot, then beeware of their colour depth and your monitor's/graphic card color depth. For example you might have set on 16 bit and the cursor being at 24 or 32, or on the contrary...that gives errors or graphic glitches!
Fiddling with the hardware acceleration helped but the problem sometimes reappeared upon reboot. Permanent fix was to turn off "trails." I use the same pointer (Windows standard) all the time - no themes.
This problem cropped up on a computer that I have been using for two years with no change in configuration. The only recent change to programs was to update Java, but I don't know if that preceded the prob.
On a separate note - does anyone know if the "Photodisc" theme is available for XP?