Unlike virus and worms, spyware is not contagious to PCs in a network. Rather than replicating itself to other PCs, spyware is happy to remain in your PC after deceiving users by surreptitiously self-installing itself on your machine. This happens often when visiting a Web site or downloading files from the net. It can take advantage of the vulnerabilities of legitimate PC application software.

Most PC users aren’t aware they have one. Once you are on the net, there’s a large probability that your PC is already infected with spyware, unless you have an Anti-spyware utility that is active every time you are online.

A growing number of freeware and shareware that you download on the net can have associated programs that behave like spyware. Spyware piggyback on legitimate software that users find useful and would install together with the software. There could be some collusion between shareware or freeware authors with spyware makers to earn money on the side.In some cases, spyware makers hijack and rewrite legitimate downloadable software to come with bundled spyware.

A few masquerades as useful programs like PC utilities, security software, web accelerators or games but are actually spyware themselves. Only when you install and scan your PC with a good Anti-spyware program do you begin to realize they are spyware.

Even non-executable files are not spared. Downloading image files, music and videos can also come with unwanted spyware. Almost anything you download on the net is game for spyware makers for them to hijack and modify so that they come with a spyware once you download it.

Spyware often exploits vulnerabilities of useful and legitimate application software, mostly notably the Internet Explorer from Windows. The easiest avenue would be simply visiting a website or clicking on a link or a pop-up window that promises a useful application like accelerating your browser. Some of the most spyware ridden sites are pornographic websites that often install unwanted pop-up ads, direct homepages to pornographic search engine sites, or install new toolbars on your browser.

They can be difficult if not outright impossible to uninstall. Most spyware latches on your computer like a leech, modifying a slew of registry items that can make it difficult to remove them manually. Only a good Anti-spyware tool could be your only hope.