mouse cursor in desktop is a blank white square - how change?

Does it happen as a new user?

Thank you for the response, Andrew. Just setting up a single new user account restores the normal arrow cursor for all users while the machine is running. After a machine restart, however, all users (including the new account) once again have the blank white square mouse cursor on the desktop.

Try this:
sudo update-alternatives --config x-cursor-theme

and restart xserver (logout-in).

Thank you for the response, Sam. The first time I tried this, I selected the default from the six options offered and the cursor appearance was unchanged after logging out and in. The second time, I selected option 6, which looked as though it's the same as the default, and, after logging out, the normal desktop cursor was restored. On restarting the machine, however, the blank white square mouse cursor returned and remains after login. BUT NOW if I log out again the proper desktop cursor returns for all users. I think this is probably new behaviour as a result of the update but I can't now be sure of the mix between restarts and logouts that I used to switch between users when I was checking out Andrew's query above.

Summarising, the current behaviour is:
on startup: blank white square cursor
after login: blank white square cursor
after logout: normal arrow cursor
after new login: normal arrow cursor

I've got two machines here running Ubuntu 11.10 64 bit AMD (both upgraded within a few weeks of each other) and only one is showing this cursor problem. I would like to check for different versions of the relevant files on the two installations but I have no idea where to start looking: any suggestions?