Malware, spyware, and other junk software makes it onto your computer for a number of reasons:

* You installed something you really shouldn’t have, from an untrustworthy source. Often these include screensavers, toolbars, or torrents that you didn’t scan for viruses.
* You didn’t pay attention when installing a “reputable” application that bundles “optional” crapware.
* You’ve already managed to get yourself infected, and the malware installs even more malware.
* You aren’t using a quality Anti-Virus or Anti-Spyware application.
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More info at the source. Excellent article if you are interested in this kind of thing.