A firewall is generally there to stop people accessing your computer. If the firewall is running on your computer then it should prevent people accessing your computer and planting viruses there. (If it is set up properly.) The internet and a LAN are more or less the same as far as a firewall is concerned.
I have heard that many viruses are spread if you create shares under windows, since they will look for shares across the network to plant themselves in. A firewall may stop this (can't confirm this) though disabling shares would be a good idea.
You would probably need a virus scanner (up to date) as well, or you could use Linux and reduce the problem greatly!
or you could use Linux and reduce the problem greatly!
i think most firewalls have different levels of security for LAN and internet...
Another reason for using Linux is that most distributions will come with a firewall installed. Price: £0.00.
Running a firewall such as the free and highly acclaimed ZoneAlarm from www.zonelabs.com will be a good way to protect your computer. But even better you want an upto date virus checker that automatically downloads the latest updates, and runs a daily scan. Also make sure your computer operating system is kept upto date with all the latest patches.