fire wall

Use a Firewall
A firewall is a piece of software or hardware that sits between your
computer and the internet and only allows certain types of things to cross
the wall. For example, a firewall may allow checking email and browsing
the web, but disallow things that are commonly not as useful such as RPC
or "Remote Procedure Calls". In fact, it's vulnerabilities in RPC that
allowed for one of the more recent worms to propagate.
What's a firewall, and how do I set one up?
Viruses like the Sasser worm can be prevented simply by using a good
firewall. What's a firewall? Well, in your car it's the "wall" of metal behind
the dashboard between you and the engine that prevents engine fires
from roasting you and your passengers.
A firewall for your computer is much the same - it's to keep you from
getting burned.
A firewall's purpose is very simple: to block or filter certain types of
network traffic from reaching your computer. What do I mean by "certain
types"? There are things you want to get like the pages of web sites you
visit or the software you might download. And then there are other things
you might not want like people accessing your computer remotely or
viruses and worms infecting your machine.
A firewall knows the difference.
Firewalls are also usually configurable; they can allow you to say "this
kind of connection from the outside is OK". A good example is Remote
Desktop. A firewall will by default prevent it from working. But you can
also configure the firewall to allow that type of connection to come
through. That way you would be able to access your computer from
another, across the room, or across the internet. But other types of traffic
like viruses are still blocked.
Some firewalls will also monitor outgoing traffic for suspicious behavior.
One characteristic of many viruses is that once you're infected they
attempt to establish connections to other computers to spread. Many
software firewalls will detect and either warn you or simply prevent it.