I had the problem Linda described on a test site (and I think it will probably work on my real site). I think it is to do with how PHP is configured, I'm no expert, but these are the changes I had to made to work around the issue:
Change
if ( ($action == 'new') && isset($HTTP_GET_VARS['prodID']) ) {
to
if ( ($HTTP_GET_VARS['action'] == 'new') && isset($HTTP_GET_VARS['prodID']) ) {
there are two places this needs doing.
If anyone has any ideas what I need to do to my PHP configuration/osCommerce configuration to avoid this issue then I'd be grateful.
TIA
Ian