Periodically downloads, programs and executables install suspicious files, or "spywhere" without user knowledge. These run in the background without permission, and frequently create undesirable "pop ups", and corrupt systems.

Here's a partial list of known offenders: here

Instant messenging tops the list, and has become a source of security concern as new viruses, worms and trojans are passed via instant messenging. Here's a site which lists level of difficutly, and how to opt out (if applicable):

here

A useful tool designed to detect and remove spyware is one from Lavasoft entitled Ad-Aware that runs on Win9x, ME, NT4.0, XP and Win2000. Be certain to use version 5.7 if within the US as no removal tool yet exists for v5.7.1. Download, run the scan, and continue to choose whether or not to remove, or block the suspected file from your registry.

Furthermore, here is a site that details spyware you may wish to check your registry for:
www.sitebilder.com/privacy/webscum.php

If you are running in a Windows environment, start/find files or folders and type in each of these components; see if they are cited above.

A final note: If your environment is Windows 95, 98, ME, NT, 2K or XP, still wish to shop, and you're not certain if the source is trusted, go here:
Anonymizer.com
Get and use the free version.

Ginger