Can not create a HSSFWorkbook.  Code compliles o.k. but I get the following 
error at run time:

Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/log4j/Basi
cConfigurator
        at net.sourceforge.poi.hssf.HSSFLog.configure(HSSFLog.java:139)
        at net.sourceforge.poi.hssf.HSSFLog.<init>(HSSFLog.java:102)
        at net.sourceforge.poi.hssf.model.Workbook.<clinit>(Workbook.java:131)
        at net.sourceforge.poi.hssf.usermodel.HSSFWorkbook.<init>(HSSFWorkbook.j
ava:127)
        at ExcelTest.main(ExcelTest.java:11)


The code that generates this error is as follows:

//~Create a workbook.
HSSFWorkbook wb = new HSSFWorkbook();

//~Name the worksheets. (Three are created by default)
wb.setSheetName(0,"First");
wb.setSheetName(1,"Second");
wb.setSheetName(2,"Third");

//~Output the workbook to file.
try{
   FileOutputStream out = new FileOutputStream("D:\\JavaSource\\excelTest.xls");
   wb.write(out);
   out.close();
}catch(FileNotFoundException fnfe){
   System.out.println("FileNotFoundException: " + fnfe);
}catch(IOException ioe){ 
   System.out.println("IOException: " + ioe);
}

Any help with this problem would be much appreciated.
Justin.