Assuming you're running Windows 98 or newer.....
Firewall - Just about any of the free versions of ZoneAlarm. The older versions are fine as a basic firewall, the newer ones have some nice extra features that you may (or may not) consider bloat.
AntiVirus - I'm running AVG version 7.5.516 on 400 mh (and older) without problems, (be sure to check for updates daily).
I'm sure others could recommend their favorite Firewall and AV as well.