A firewall is a particularly important device for broadband users who leave their PC on-line all the time.

These machines are particularly vulnerable as they are useful to hackers who want to "own" your PC so they can launch attacks from it and thousands of other drone PCs they have captured.

Windows XP with SP2 has an in-built firewall, however, some people think it is not good enough - and given Microsoft's record for keeping hackers out they are probably right.

A great and free to home-users firewall is ZoneAlarm. This can be downloaded here: http://www.zonelabs.com.

This firewall software will stop you getting "owned" and if you are already "owned" it will stop any already resident deviant software from obeying its master's command.