rename() definitely does not follow the *nix rename convention on WinXP with PHP 5.  If the $newname exists, it will return FALSE and $oldname and $newname will remain in their original state.  You can do something like this instead:

function rename_win($oldfile,$newfile) {
   if (!rename($oldfile,$newfile)) {
      if (copy ($oldfile,$newfile)) {
         unlink($oldfile);
         return TRUE;
      }
      return FALSE;
   }
   return TRUE;
}