Hi,
Try these two commands in the console as root:
rm -rf /var/lib/rpm/__db*
rpm --rebuilddb
Once it gives the prompt back to you, in the same root console do:
apt-get update
and
apt-get install pcmanfm
Well if you are sure that the distro is fully up to date, of course. If not, after "apt-get update" you can also type "apt-get dist-upgrade" for a full update of the system.
Hopefully it should work. If not you could get the necessary packages directly from the repos, but not having the package manager working normally would be strange.