Datapump doesn't work on 11g XE on Windows (didn't try on Linux), you get something like that:
Export: Release 126.96.36.199.0 - Beta on Wed Aug 31 22:30:25 2011
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Connected to: Oracle Database 11g Express Edition Release 188.8.131.52.0 - Beta
ORA-39006: internal error
ORA-39213: Metadata processing is not available
It occurs because there are a entire directory missing:
If you copy that directory from another 11gR2 installation (even a Linux one) to your 11g XE and execute the following as sys: