In the 1970s, ARPANET was designed so that scholars could exchange research papers. Since federal grants funded its growth, commercial traffic was forbidden. It was designed for use by trusted people in white lab coats and little thought was given to security.
ARPANET became the Internet and the rules changed. Tim Berners-Lee's creation of the World Wide Web in 1991 provided the spark that ignited the Internet. The majority of traffic today is commercial. The downside to this flow of dollars is that it's attracted hordes of thieves. They try to steal your money, your privacy, and even your identity.
One of the tricks of their felonious trade is to fool you into installing a destructive program on your computer. Often this is done with trojan horses: the program looks benign, but it hides a nasty payload.
Ten years ago, viruses and worms were either relatively harmless or just randomly malicious. Today there's a profit motive that drives clever programmers to create keyloggers, rootkits, bots, spyware, trojan horses, and adware.
If you think, "No one would bother to attack my PC. There's nothing on it that's valuable", you're wrong.The attacks are launched by programs that scan the Internet, looking for vulnerable computers. They'll attack your PC simply because they can. I've found really nasty infections on computers that are owned by retired couples and little old ladies from Pasadena (well . . . Pompano Beach).
Here's how I treat infected computers
First, I identify the PC infections: spyware, bots, rootkits, viruses. Then I eradicate them. This requires a variety of techniques, as virus scanning programs return false negatives; sometimes they're unable to remove viruses that they do identify. Only once a system passes at least four tests do I pronounce it clean of infection.
Patience (from me -- not you) is required to completely remove viruses from PCs.
Then I harden the computer. This process protects the computer from ever communicating with thousands of known malicious sites. Then I toughen Internet Explorer's defenses, install Mozilla Firefox with add-ons, and install high value anti-virus and anti-spyware software and firewalls.
If your PC is slow, you can't visit favorite websites, your home page has changed, unwanted ads pop up, or you're told that an infection has been found and you must buy a program that will disinfect it, your computer is infected, Call me: