Spyware can closely resemble computer viruses, but with some important differences. Many spyware programs install without the user's knowledge or consent. In both cases, system instability commonly results.
A virus, however, replicates itself: it spreads copies of itself to other computers if it can. Spyware generally does not self-replicate. Whereas a virus relies on users with poor security habits in order to spread, and spreads so far as possible in an unobtrusive way (in order to avoid detection and removal), spyware usually relies on persuading ignorant or credulous users to download and install itself by offering some kind of bait. For example, one typical spyware program targeted at children, Bonzi Buddy, claims that:
"He will explore the Internet with you as your very own friend and sidekick! He can talk, walk, joke, browse, search, e-mail, and download like no other friend you've ever had! He even has the ability to compare prices on the products you love and help you save money! Best of all, he's FREE!"
[1] (http://www.bonzi.com/bonzibuddy/bonzimail.asp)
A typical piece of spyware installs itself in such a way that it starts every time the computer boots up (using CPU cycles and RAM, and reducing stability), and runs at all times, monitoring Internet usage and delivering targeted advertising to the affected system. It does not, however, attempt to replicate onto other computers - it functions as a parasite but not as an infection. Spyware generally does not damage the user's data files; the overwhelming majority of the harm inflicted by spyware comes about simply as an unintended by-product of the data-gathering or other primary purpose.
A virus does deliberate damage (to system software, or data, or both); spyware does accidental damage (usually only to the system software). In general, spyware cannot damage the computer hardware itself. Certain special circumstances aside, in the worst case the user will need to reformat the hard drive, reinstall the operating system and restore from backups. This can prove expensive in terms of repair costs, lost time and productivity. Instances have occurred of owners of badly spyware-infected systems purchasing entire new computers in the belief that an existing system "has become too slow." If you believe your computer is "slowing down" (as opposed to "becoming outdated") you might consider installing and running anti-spyware software such as AdAware or SpyBot Search and Destroy.