Nick is on a Mac, which is immune to all Windows viruses. Firewalls are a superb safety item too.
Jeremy, if you have your own firewall (most routers will do the job) you know it's working. I wouldn't trust my cable ISP.
As for automatic installs, you must have automatic update turned on, and/or you've already got bad software running that's doing nasty things to you. Clean it up, install a cheap router, and you'll be safer.
ISPs have traditionally NOT blocked the TCP ports that caused the recent Windows problems...so much for their firewall.