We believe that the Internet is free. That means free stuff to download. That means free stuff to use. That means free email. Unfortunately this means that the odds of a boomerang coming back are the same as spyware coming back to your computer.
Spyware can also come bundled with shareware or other downloadable software. The user downloads a program and installs it, and the installer additionally installs the spyware. Although the desirable software itself may do no harm, the bundled spyware does. In some cases, spyware authors have paid shareware authors to bundle spyware with their software. In other cases, spyware authors have repackaged desirable freeware with installers that slipstream spyware.
Spware Info: Why does spyware keep coming back?