Industry experts suggest that these types of programs may infect up to 90 percent of all Internet-connected computers. Typically, spyware arrives bundled with freeware or shareware, through email or instant message, or by someone with access to a user’s computer.

What is spyware and why should Internet users be concerned?
• Adware is advertising-supported software that displays pop-up advertisements whenever the program is running. Often the software is available online for free, and the advertisements create revenue for the company. Although it's seemingly harmless (aside from the intrusiveness and annoyance of pop-up ads), adware can install components onto your computer that track personal information (including your age, sex, location, buying preferences, or surfing habits) for marketing purposes.
Some advertising-supported software will not inform you when it installs adware on your system, or will bury such notification in small print. In many cases, the software that is financially supported by adware will cease to function without the adware component. Sometimes adware will infiltrate your computer even when you decline the installation.
• Adware cookies are pieces of software that Web sites store on your hard drive when you visit a site. Some cookies exist just to save you time-for example, when you check a box for a Web site to remember your password on your computer. But some sites now deposit adware cookies, which store personal information (like your surfing habits, usernames and passwords, and areas of interest) and share the information with other Web sites. This sharing of information allows marketing firms to create a user profile based on your personal information and sell it to other firms.
Adware cookies are installed and accessed without your knowledge or consent.
• System monitors can capture virtually everything you do on your computer, from keystrokes, emails, and chat room dialogue to which sites you visit and which programs you run. System monitors usually run in the background so that you don't know you're being watched. The information gathered by the system monitor is stored on your computer in an encrypted log file for later retrieval. Some programs can even email the log files to other locations.
There has been a recent wave of system monitoring tools disguised as email attachments or free software products.
• Trojan horses are malicious programs that appear as harmless or desirable applications. Trojan horses are designed to steal or encode computer data, and to destroy your system. Some Trojan horses, called RATs (Remote Administration Tools), give attackers unrestricted access to your computer whenever you're online. The attacker can perform activities like file transfers, adding or deleting files and programs, and controlling your mouse and keyboard.
Trojan horses are distributed as email attachments, or they can be bundled with other software programs.