Yes, you need to be security conscious. It isn't just about viruses, but
identity theft, etc. I am no security expert, so I will let someone better
versed than I to answer the hows and whys of hardening your system.

However, updates are a necessary part of any operating system. It reflects the
changes that happen as it is constantly being tweaked and perfected. There are
kernel developers who are constantly improving hardware detection making sure
that people who buy new equipment can install Linux. There are security
developers who examine threats as new ones constantly emerge and find ways to
guard users. This is referred to as hardening the system. There are new things
everyday as new features are added to the desktops, new video and wireless
drivers as more OEMs support Linux, and new projects that keep a different
schedule from your Linux developers. Just recently a new OpenOffice came out for
example with exciting new features. The package managers want to make this
available to you so they updated OO in the repositories. A new version of the
GIMP is out and it is available for download. The old one works fine, but the
new one has new features that some people
have been demanding. So it must be updated if you want to have these features.
The only way to avoid updates is to make a conscious decision that you are happy
and want to be stuck in time. If you crave stability then this is in effect what
you are doing. It is a tradeoff that you must make. People on the other extreme
who want to be on the bleeding edge tradeoff stability for having the newest,
latest and greatest features. Most users find some place between these two
extremes. They can control what level they want and this is something that Linux
does promise. It is infinitely flexible, configurable and offers real choice.