please run this in the terminal and please copy and paste posy the out put here.

sudo aptitude update

It looks like it updated just fine did you get at the end of the update?

Reading package lists... Done

Then goes back to the $ prompt?
What program are you installing? Are you installing through Synaptic or by the
terminal. If you are installing from Synaptic. Try running the want to be
install program through the terminal and see what error you get.