Go into the Tools menu, select Customize then click on the Keyboard tab. In the Shortcuts box the Down key should have Move Down as its function. Also click on the OpenOffice.org radio button and check whether Down is either set to Move Down or has nothing next to it.
To fix the problem, you could try Reset, but that would clear other keyboard settings you may have. To affect only that one, click the Calc radio button, select Down in the Shortcut Keys box (for Calc, the Modify button may be grayed out - click the Delete button). Now select Internal in the Category box, Move Down in the Functions box and click the Modify button. Now click the OK button to save (or the Cancel button to cancel).
If the setting for Calc is OK and the setting for OpenOffice.org is not (or it's blank) then that will override Calc's setting. Perform the steps above but click the OpenOffice.org radio button instead of the one for Calc.