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The public good, is in nothing more essentially interested than in the protection of every individual's private rights. - Blackstone
Programs generically called spyware invade your privacy by secretly sending back information about how you use your PC to their masters. How this information is used may or may not be in your best interest. But be assured it is in the best interest of advertisers and perhaps the government.
To combat this, there are a couple of free programs that scan your PC looking for spyware. One is called Spybot and can be found here. The other is Ad-aware (see their site here) and of the two, is probably the better known. Ad-aware recently came out with version 6.0 so if you haven't updated yet, you may want to head over to their site. But whether you choose Spybot or Ad-aware, you probably should be using one of them.
Over and above any privacy issues is the practical one of CPU usage. These spyware programs can consume much of your CPU's time, thus dramatically slowing everything else you want your PC to do. So if your PC seems to have slowed to a crawl, and you've ruled out the usual suspects of low disk space or memory, then perhaps you might want to download and install one of these.
Aloha!