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NoClassDefFoundError org.apache.poi.ss.usermodel.Cell with Excel 2007

Hi guys,
when trying to load an Excel 2007 file using TOS Version: 4.2.2 Build id: r63143-20110622-0628 on Windows 7 32-bit I get the following exception:
java.lang.NoClassDefFoundError: org/apache/poi/ss/usermodel/Cell
Caused by: java.lang.ClassNotFoundException: org.apache.poi.ss.usermodel.Cell
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
Could not find the main class: xxx.common_data_0_1.common_data. Program will exit.
Exception in thread "main"
Any idea what's wrong here?
Thanks!
Chris