The problem with Mint is that it is a derivative and it has some of its own
repositories and it also shares Ubuntu's. The net is full of people who have had
problems with Mint and its repositories. One problem is that their site goes
down frequently. This can leave you with a partial update and cause error
messages. Another is that if you start thinking that Mint is Ubuntu then you can
cause it to behave that way and you lose the Mint side of things. Really what
you have is Mint with the GNOME desktop so don't install anything that says
Ubuntu desktop. It is okay if it says GNOME desktop.

You say that you boot up to a CDROM. Does that mean that you are running Mint
from the CD and it isn't actually installed? If you think that it is good why
wouldn't you install it? If you are running from the CD then you can't update
anyway since any changes you make will be lost when you re-boot. If it is
installed why do you need the CD?

Other than this (it not being installed), I am not sure what your exact problem
is. I think that you should
consider upgrading to Felicia (Mint 6), since your system is not
working properly anyway. However, that is just the way I think. It is
always better to have the latest and greatest. Some people disagree and
want to play it safe and only use the LTS releases, which for Mint was
Elyssa (5.1).

To install Mint is really quite simple, however, we should know a bit more about
your system, such as how many drives, partitions and what other OSes are
present?