unable to install applications. unhandled error occured

[james lumb]

Unable to install any applications. when attempted, an box appears with "An unhandled error occured"
Details:

Traceback (most recent call last):
  File "/usr/lib/python2.6/dist-packages/aptdaemon/worker.py", line 768, in simulate
    return self._simulate_helper(trans, status_path)
  File "/usr/lib/python2.6/dist-packages/aptdaemon/worker.py", line 938, in _simulate_helper
    return depends, status, self._cache.required_download, \
  File "/usr/lib/python2.6/dist-packages/apt/cache.py", line 218, in required_download
    pm.get_archives(fetcher, self._list, self._records)
SystemError: E:I wasn't able to locate file for the libqt4-network package. This might mean you need to manually fix this package.