Apt-get question (reverting a install)

[Didster <didster@gmail.com>]

Hi All,

I have a quick question RE apt-get.  I am a newbie.  I recently tried
to install a package only available in unstable on a stable system. 
Being the fool I am, I did this before reading up on it, and just
added the deb lines for unstable to the sources.list file – without
setting any pinning options or setting APT::Default-Release.  When I
did apt-get install <newpackageonlyinunstable> apt went away and got
it, plus its dependences from unstable.  Some of these dependences
where also dependant on other things which were available in stable –
but apt got them from unstable – the net result was basically 40 or so
packages being upgraded to unstable, when they didn't really need to
be.  The system now takes 5 minutes to boot, as apposed to 30ish
seconds before – and takes over a minute to start syslogd.

 I'm trying to figure out a way of rolling this back to what it was
before.  When I do apt-show-versions it shows all the unstable
packages.  First, I tried removing the unstable lines from
sources.list, then doing apt-get update.  When I have done that, and I
do apt-show-versions it now says "No Version Available" for all the
unstable packages.  An apt-get install on any of them just says sorry,
you have the latest version.  An apt-get install package=version seems
to work, but basically says its going to remove half the system (one
of the packages upgraded to unstable was libc6…)

Does anyone have any idea how I can fix this?  Or should I just reformat?

While I am here, could someone explain how apt's lists and cache work
with or use dpkgs available/status files??  I kind of understand them
separately, but am having trouble getting my head round how exactly
apt interacts with dpkg.  And also, what APT::Default-Release really
does?

Thanks a lot.