Unfortunately there is a limit of three computers allowed per subscription. Two accounts with different Live IDs would be required to protect five computers with One Care. Forefront would not be an appropriate solution for a small home network even with a server, which would be a requirement -
http://www.microsoft.com/forefront/clientsecurity/en/us/pricing-licensing.aspx
Jim - MVP Windows Live - Forum Moderator - Live One Care - Live Mesh