Apache PHP and temporary files

We are using apache with php on winndows 2000. When we use the dynamic image functions, we get the following message: "imagepng: unable to open temporary file" This problem goes away if we give our users the wright to use the directory "c:\". But we would prefer not to give them access to this directory for security reason. Anybody knows how we can tell Apache or Php to write the temporary files to another directory. Thanks for your help Sarto Beaudoin