Free software can help protect your PC from software that secretly invades your privacy or bombards you with unrequested advertising...
Spyware and adware explained
According to Wikipedia:
Strictly defined, spyware is computer software that gathers information about a computer user without the user's knowledge or informed consent, and then transmits this information to an organisation that expects to be able to profit from it in some way. Data-collecting programs installed with the user's knowledge are not, properly speaking, spyware, if the user fully understands what data is being collected and with whom it is being shared.
More broadly, the term spyware is applied to a wide range of related malware products which are not spyware in the strict sense. These products perform many different functions, including the delivery of unrequested advertising (pop-ups in particular), harvesting private information, re-routing page requests to illegally claim commercial site referral fees, and installing stealth phone dialers.
According to Wikipedia:
Adware or advertising-supported software is any software application in which advertisements are displayed while the program is running. These applications include additional code that displays the ads in pop-up windows or through a bar that appears on a computer screen. Adware helps recover programming development costs, and helps to hold down the price of the application for the user (even making it free of charge)—and, of course, it can give programmers a profit, which helps to motivate them to write, maintain, and upgrade valuable software.
For more discussion on the rise of adware and spyware, see this article from Wired.
Detect and destroy spyware using Spybot
Spybot describes itself thus:
Spybot—Search & Destroy can detect and remove spyware of different kinds from your computer. Spyware is a relatively new kind of threat that common anti-virus applications do not yet cover. If you see new toolbars in your Internet Explorer that you didn't intentionally install, if your browser crashes, or if you browser start page has changed without your knowing, you most probably have spyware. But even if you don't see anything, you may be infected, because more and more spyware is emerging that is silently tracking your surfing behaviour to create a marketing profile of you that will be sold to advertisement companies. Spybot-S&D is free, so there's no harm in trying to see if something snooped into your computer, too :)
Detect and remove adware using Ad-aware
Ad-aware describes itself thus:
Ad-aware Standard Edition is THE award winning, free, multicomponent detection and removal utility that consistently leads the industry in safety, user satisfaction, support and reliability. With its ability to comprehensively scan your memory, registry, hard, removable and optical drives for known datamining, aggressive advertising, and tracking components, Ad-aware will provide the user with the confidence to surf the Internet knowing that their privacy will remain intact. Let Ad-aware protect your privacy.
We have added a removal feature for some of the symptoms you mentioned [http://spyware.removal.nospyx.com/free/spyware-scan/] If anyone wants a free scan
Yup. All great tools. Yahoo recently released one in beta, too, that's part of the Y! Toolbar, but it was only available for limited downloads during the beta period.