As you have Norton Anti Virus you will qualify for 'upgrade' to Norton Internet Security 2004 (has some useful additional functions over 2003) for about GBP 29 (in the UK)
Have after testing on one machine am now installing on half a dozen or so used by collegues, having done a second one to solve a nasty 'pop up' problem.
You definately need to move to firewall protecvtion ASAP.