Possible Duplicate:

Cannot install any software from the Software Center due to ttf-mscorefonts-installer package error

I was trying to install ubuntu-restricted-extras from the command line, as instructed here: Can't play dvd with movie player

Near the end, I came to a EULA screen that wouldn't let me past it. Clicking OK, hitting enter, nothing worked.

After aborting that attempt at installing, I find that attempting to install anything leads to the following message:

Traceback (most recent call last):

  File "/usr/lib/python2.7/dist-packages/aptdaemon/worker.py", line 961, in simulate

trans.unauthenticated = self._simulate_helper(trans)

  File "/usr/lib/python2.7/dist-packages/aptdaemon/worker.py", line 1085, in _simulate_helper

    return depends, self._cache.required_download, 

File "/usr/lib/python2.7/dist-packages/apt/cache.py", line 226, in required_download

pm.get_archives(fetcher, self._list, self._records)

SystemError: E:I wasn't able to locate file for the ttf-mscorefonts-installer package. This might mean you need to manually fix this package.

(The above was an attempt to install 7zip from the Software Center.)

How do I undo what I've done? Tell me how this can be fixed without re-installing the OS.

I'm not particularly Linux-savvy, so please help me to understand how I can investigate this problem, and hopefully regain the ability to install software without reinstalling the OS.