Avast! Free Antivirus is a popular and very efficient choice. It protects against viruses and spyware with realtime scanning, including email and websites.http://www.avast.com/free-antivirus-mac
Sophos is another good free one right up there near the top, and it is lightweight, with good detection rates, and in my testing has never caused any issues.http://www.sophos.com/en-us/products/free-tools/sophos-antivirus-for-mac-home-edition.aspx
As far as a firewall goes. The one included with OS X 10.6 is perfectly usable. It is an Application Firewall, which allows you to control connections on a per-application basis, rather than a per-port basis. This makes it easier to gain the benefits of firewall protection, and helps prevent undesirable applications from taking control of network ports that have been opened for legitimate applications.
Follow these steps:
Choose System Preferences from the Apple menu.
Click the Firewall tab.
Unlock the pane by clicking the lock in the lower-left corner and enter the administrator username and password.
Click Start to enable the firewall.
Click Advanced to customize the firewall configuration.
I should note, however, that I myself do not run with any real time third party scanning protection at this point. There just aren't a lot of threats out there right now that can't be avoid using safe internet practices. i say this not to discourage you from exploring the above options BUT if you find they are troublesome or effect your workflow then you shouldn't feel you MUST use them, IMO.
If you still have concerns, simply reply back before rating. I'll be more than happy to work with you further. Hopefully, this info will answer your question. When you are satisfied, please give my answers a positive rating. You are judging my personal service only (not this website or your issue) and positive ratings assure I remain in good standing.
Thanks and good luck,