Linux, being a unix-like OS, is unaffected by pc viruses. Of course,
nobody is claiming that linux is invincible, but it's a much harder
target for crackers. Linux users generally don't run any anti-virus
software and don't need to. Some shell shocked refugees from the
microsoft world sometimes insist on having anti-virus and a disk defrag
utility, neither of which have much use in the linux world.

The ubuntu website has some good user guides and the forum there is helpful.