Obrigada Cláudio,
Segui as suas dicas como vc postou e pela mesma ordem, mas não sei se fiz certo. Mas o erro sumiu, o que já é muito bom!
Se não for pedir demais veja ai como ficou o terminal depois que fiz tudo o que me indicou:
laptop:~$ sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 165d673674a995b3e64bf0cf4f191a5a8844c542
Executing: gpg --ignore-time-conflict --no-options --no-default-keyring --secret-keyring /etc/apt/secring.gpg --trustdb-name /etc/apt/trustdb.gpg --keyring /etc/apt/trusted.gpg --recv-keys --keyserver keyserver.ubuntu.com 165d673674a995b3e64bf0cf4f191a5a8844c542
gpg: requesting key 8844C542 from hkp server keyserver.ubuntu.com
gpg: chave 8844C542: "Launchpad PPA for xorg crack pushers" sem alterações
gpg: Número total processado: 1
gpg: não modificados: 1
laptop:~$ sudo gedit /etc/apt/sources.list
laptop:~$ sudo aptitude update && sudo aptitude safe-upgrade
A gravar informações extendidas de estado... Pronto
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Obteve 96,5kB em 1s (61,8kB/s)
A ler as listas de pacotes... Pronto
W: Erro GPG:
http://ppa.launchpad.net jaunty Release: As seguintes assinaturas não puderam ser verificadas porque a chave pública não está disponível: NO_PUBKEY 6AF0E1940624A220
W: Duplicate sources.list entry
http://packages.medibuntu.org jaunty/free Packages (/var/lib/apt/lists/packages.medibuntu.org_dists_jaunty_free_binary-i386_Packages)
W: Duplicate sources.list entry
http://packages.medibuntu.org jaunty/non-free Packages (/var/lib/apt/lists/packages.medibuntu.org_dists_jaunty_non-free_binary-i386_Packages)
W: Você terá que executar apt-get update para corrigir estes problemas
Estado actual: 18 actualizações [+18], 12 novos [+12].
laptop:~$
O alerta de atualizações avisa novas atualizações! Estou com esperança que ao menos esta fase esteja superada. Agora falta ver como resolvo o resto... área de trabalho que não aparece nem com reza, Rmvb que roda defasado no Mplayer, VLC, Xine...
Mas muito obrigada mesmo!!!