Sheldon,

A virus checker attempts to detect, intercept, and destroy/isolate a program meant to do mischief or harm on your computer. A firewall attempts to intercept/prevent incursions to your computer system by a foreign computer via your internet connection. Sometimes the incursion will attempt to find a virus or worm already planted on your computer via email, freeware, etc. If found, it may be used either to control your computer remotely , use your computer to control other computers, or do other mischief or damage.

This explanation is probably oversimplified, but I'm a user, not a techie. Sometimes I know just enough to be dangerous, but so far I've managed to keep a clean computer. That doesn't mean that nobody will ever get the jump on me, that's why I try to be so vigilant. Hope I've helped.

Kerry