Eclipse 3.2RC7

I am having some problem when I build the ant file (build.xml)

Badd class file: C:\Program Files\Java\jre1.5.0_07\lib\rt.jar(java/io/Serializable.class)
class file has wrong version 49.0, should be 47.0

I read this Malcolm Davis's Blog, it says 'Eclipse 3.1 looked rushed'. Version 3.2 seems to have a lot of changes, so I try 3.2 from eclipse download, let see the version 3.2RC7 will solve my problem.