One of the more recent additions to the plethora of Web-caused problems is spyware. Spyware is a very general term that can refer to a lot of things, many of which could be considered viruses or trojans by some definition. The worst type of spyware is that which actually records your personal information - such as your credit card information, email password, address, etc. Thankfully, this is actually the least common of spyware. A more common type of spyware you'll find simply tracks your browsing habits and uses that information for either statistical purposes or to provide you with ads, which leads me to the most common type of spyware, adware. Whenever you see a pop-up (or if your browser or blocker stops it), it's probably spyware.
Use a router for improved security - How much?
Spyware can infect your computer in a number of ways: It can be a program that you knowingly installed; it can be a cookie in your web browsers; it can be a browser plug-in or ActiveX control you installed, knowingly or not; it can be a program that appeared fine, but installed spyware of some sort without telling you (these programs are known as trojans, by the way). In any case, spyware, like viruses, are often best dealt by simply entrusting a program.
To prevent and remove spyware, I recommend you use either or both of two programs: Ad-Aware SE and Spybot Search and Destroy. Both are free programs that provide excellent scanning abilities which will remove most spyware. You can pay to get some additional preventative properties, but I've found that these are mostly useless and that you'll just be wasting your money. Get one or both programs and update them and scan with them often.