how to secure the system

I would like to know how to secure my ubuntu seystem , I am using ubuntu 11.04 , how can I secure it from hacking?????????????
I disabled blue tooth and there is no wifi, whichelse the way I keep monitoring the system, can I see the entries if
hackers enter my system??

I using browser epiphany of ubuntu is there any chance of hacking this?

one morething I would like to know if I upgrate it my previous setting lost or it will be upgraded with same setting???
like I disable desktop viewer and guest log in?

looking for your prompt answer .

thanks

Gnu/Linux (and then Ubuntu) is one of most security system in the world. All you have to do is just to stay calm. You haven't something special to do for protect your system, is secure alredy now. The viruses on Linux are really few and they don't work now because they are too old and the problem that they used was solved, here is a list:
https://help.ubuntu.com/community/Linuxvirus

Read also this, it's interesting:
http://librenix.com/?inode=21