Risks of File Sharing

My two computers are set up to share files over my home network. The network is firewalled and Zone Alarm has been set to permit the two computers to communicate. The sharing only goes one way (ie slave to master).

Question - are there risks associated with the settings required to share files which could permit a virus to gain access - or are the firewall and ZA Trusted Zone sufficent to protect both computers?

Thanks in advance for your advice.

AK