I run POI on Windows 2k SP 3 without any problems.

Such an error is almost certainly due to not putting 'poi.jar' on the 
classpath, then trying to use a static method. Try putting a debug statement or 
log statement in CreateParamXLS.writeWorkbook(), immediately before and 
immediately after the first access to a POI object. Like this:

   System.out.println( "About to create Workbook");
   Workbook myWorkbook = Workbook.createWorkbook();
   System.out.println( "Workbook created");

I bet this will show the first line, but not the second.
If that's your problem, it really isn't a bug.

If you are writing a servlet, remember that poi.jar has to be in the classpath 
of the servlet container. The best way to do this is to make sure that the file 
is in WEB-INF\lib of your .war file.

Not a poi bug, sorry.