Can a virus pass from one computer to another when both are connected to a router?

The XP Firewall (and other firewalls) will protect only the computer on which it is enabled, so if you wish to protect both computers then please do enable it on both (Control Panel -> Windows Firewall). In fact, it is on by default on most installs of XP.

It may be useful to note what a firewall does and does not do:

A firewall protects your computer from a specific type of attack - where another PC on the network may be trying to attack your computer, either due to a virus or a malicious hacker. In this case this is exactly what you need because you're asking about protection from the other computer. The firewall does this by rejecting any attempts by the other PC to talk with your computer. Think of the firewall as a locked front door, it keeps out burglars and neighbors when you are not home.

But note the limitations of a firewall:

The firewall does not protect your computer from things you yourself might do. For example, if your click on an unknown email attachment, or if you download an infected freeware/shareware/p2p program and run it, or if you visit a phishing web site and give them your credit card information etc., the firewall offers no protection for such cases. Think of this as you opening the locked front door from the inside. The best protection for such cases is your own vigilance, backed up an updated AV program and an anti-malware program, with greater emphasis on your own vigilance.

If you already have the Norton firewall enabled on one of the computers then you need not enable the XP Firewall on that one, but there is no harm in enabling both on the same PC if there is any doubt.

Good luck.

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