Hi Tony,
good to hear that u choose UBUNTU
welcome to the world of unlimited choise
to play videos there are so many players including VLC(my personal
fav)
please go through the list of players in one of my recent threads
http://groups.google.com/group/ubuntu-user-community/browse_thread/thread/7e517f1baec9beca
since you are using ubuntu 9.04 follow the steps below to install vlc
Open Synaptic (System -> Administration -> Synaptic Package Manager).
In Settings -> Repositories,
make sure you have a "multiverse" repository activated. then click on
reload
Search for vlc and install it. You should also install vlc-plugin-esd,
mozilla-plugin-vlc (and libdvdcss2).
coming to the second part of your question
you dont need any anti-virus software in linux
this is the world of freedom, world with out windows
even if you get a virus infected file/media, feel free to open it
the virus will not effect the machine under ubuntu(linux)
have fun
-Che