The Vista firewall isn't anti-virus. She will need an antivirus at least. The Vista firewall is better than the XP one and may be sufficient as a firewall. There are several threads on free antivirus software, firewalls, malware, etc.
You could look at AVG, Avast, Microsoft's new product and so on. For mal ware Malwarebytes, Spybot are just 2.