I realized this in spades as my older daughter's
PC recently ground to a halt. Naturally, I
suspected the cause to be tons and tons of spyware
and viruses, and sure enough I was right: Not only
did I run Ad-Award, PestPatrol and ZoneAlarm, but
I even downloaded and ran Spy Sweeper (thanks for
that tip, Walter Mossberg), which came up with
gunk no other software found.