Online security threats come in many forms — the most common types come from spyware and viruses, which could install themselves without your knowledge as you visit Web sites or download software. Spyware programs are specifically designed to infiltrate your computer for commercial gain (stealing personal information or displaying annoying advertisements), while viruses are typically designed to damage your computer.

Types of spyware programs include system monitors that capture your e-mails and keystrokes, Trojan horses that can steal or destroy data, adware that pops up advertising, and tracking cookies that store information about your online preferences and habits. While some of these programs may be harmless, others can steal your personal information and send it to a third party for malicious purposes. In some cases, these programs can damage your computer enough to slow down processing times or cause system crashes.

Spyware and other unwanted programs may be part of a program that you installed or they may install themselves as you visit Web sites. They could also arrive bundled with freeware or shareware, through e-mail, or by someone with access to your computer. These programs are difficult to detect, and difficult (if not impossible) to remove. Spyware can hide in multiple locations and can reinstall if you don’t remove it properly with specialized antispyware software like Spy Sweeper.

A virus is computer code that often duplicates itself and causes a specific event to occur. The event may be harmless, such as displaying a message on a specific date, or may be malicious, such as deleting data or duplicating data to fill a hard drive. Viruses can arrive as file attachments to e-mail, as embedded files on a CD, and as clickable graphics in an e-mail. Once on your hard drive, viruses are difficult to detect and remove, unless you have specialized antivirus software.