Once upon a time on this site, I recommended McAfee's virus-checking programs. Let the record show that the McAfee people have "improved" their Active Shield product a number of times in the last year and, with each "improvement," it gets slower and clunkier and more likely to crash my system. Ergo, Active Shield is no longer on my system. No McAfee product is. I've switched over to Norton Anti-Virus and, so far, it's working great.
If you don't have a virus-checker installed, you should. And if you do have one, you oughta know about a tiny program called eicar.com. This is a perfectly harmless fake virus that was written to test virus-checkers. You can download it here…and if you run it and it doesn't trigger your virus-checker, you ain't got no protection.