Check out this website for the lowdown. Foistware/Spyware

Unlike viruses or worms, spyware isn't necessarily dangerous. Most
spyware takes one of two forms: surveillance software, which tracks
your behavior while you use your computer, and adware, which barrages
you with advertisements when you least expect them.

Of the two, surveillance spyware is of greatest concern, because you
don't really have any idea who's seeing your personal information or
what they're doing with it. Adware, by contrast, is mostly just
irritating -- unless it's advertising unsavory or offensive content.

Fortunately, a number of resources and tools can help you detect and
eliminate spyware. If you're concerned about your privacy, or are
experiencing an unexplained increase in the number of pop-up ads you've
been seeing recently, check out the resources below.

Ad-Aware
http://www.lavasoftusa.com/support/download/
Easy to use program that identifies spyware and eliminates it for you.

SpywareInfo
http://www.spywareinfo.com/
Articles, links, resources and other useful information to protect
yourself from spyware or remove it if your computer is already infected.

Spychecker
http://www.spychecker.com/
Check before you download: Spychecker is a database that indexes
almost a thousand software titles using spyware or adware.

Bugnosis
http://www.bugnosis.org/
A free utility from the Privacy Foundation that detects "web bugs,"
code that can be attached to "invisible" web graphics on a web page
and used to track your browsing activity.

Free Popup Killer Tools
http://www.webattack.com/Freeware/misctools/fwpopblock.shtml
Reviews and free downloads for a wide range of pop-up killer tools.

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