When adware started as a business model, most people really didn't like it. They saw it as a threat to the future of free software, and actively campaigned against it. Today, however, many people are more accepting of adware, because they've turned their attention to something even worsespyware.

Spyware is adware that monitors your computer to see what you dowhich websites you visit, what you're interested in, what you buy, and more. It then sends this information back to a central database, and the spyware company profits from it in some way, often by selling the database on or by using it to send you targeted advertising. Obviously, people aren't too happy about this, especially when it is going on behind their back.

Even more often than adware, spyware will be installed by an unsuspecting user, often using Internet Explorer's ActiveX download mechanism. It is estimated that up to nine out of ten computers connected to the Internet today may be infected with spywareroughly as many as are using Internet Explorer.

Many toolbars that you can install into Internet Explorer are actually spyware, as they can easily see which websites you go to and report backAlexa, for example, uses its toolbar to compile rankings on the popularity of different websites. More malicious spyware may insert affiliate codes into web addresses when you are shopping, so that stores like Amazon.com end up sending them referral fees, even though they didn't really refer you to the store.

Thankfully, programs that started off removing adware now remove all the different kinds of spyware as well, so if your computer is infected, you just need to go and grab Ad-Aware or Spybot and do a scan. You might also want to install a personal firewall, which will alert you if any programs try to send data over the Internet without your permission.