Yes, you most definitely need antivirus software, and a firewall is advisable. I use BT broadband, BT are currently running a service where they scan your emails for viruses before you get them, and it works.
I am currently using zonealarm for firewall purposes, it is free, but as it is a software firewall, it is not perfect, however there are a number of hardware based adsl routers for sale which combine a firewall with a switch, allowing you to share your broadband connection between more than one PC, if you so wish. They are available from
http://www.zionmedia.co.uk