The idea of eternal conscious torment in hell is a doctrine invented by the Roman Catholic Church. I personally believe that unbelievers will fry for whatever amount of time is appropriate for their sins (God is a just god), and then they will probably be annihilated (i.e. cease to exist), which is for all eternity.

The idea that all souls are eternal comes from Platonism. According to Scripture only those who believe in the Messiah will live for eternity. So I don't think those in hell will also have an eternal existence.