Spyware & Malware
This stuff's the bane of PC users these days. Primarily it infects computers via web browsers, especially Internet Explorer, using scripting languages like ActiveX or Javascript. Sound greek to you? Relax, scripting is what gives websites their multimedia action. But it adds to your vulnerability, thus the horror of drive-by spyware installs. Another increasingly common way to infect a PC with spyware is via instant messaging (aka IM). Yahoo, Google, MSN, AOL and others all offer IM services. Like email, users are offered file attachments or transfers that are infected. More here...
Handling Spyware
Spyware behave similarly to viruses. In fact, the distinction is becoming quite blurred. Unfortunately for users, anti-virus programs don't handle spyware very well (AntiVir may be an exception) and you'll need a specialized app to handle spyware. There's some good anti-spyware programs out there that are up to the job. "Free" programs such as SpyBot (donationware, actually) and Ad-Aware (also offer a feature-rich paid version) are probably the best if you're on a budget. In fact, SpyBot is very good by all standards. Of the paid programs, Spy Sweeper from WebRoot may be the best. Spyware Doctor is also very good. There's plenty of offerings, free and otherwise, out there for the taking.
Buyer Beware
There's plenty of supposed anti-spyware products that are of unknown, questionable, or dubious value as anti-spyware protection. Some even resort to to generating ridiculous false positives in order to trick you into buying their product. They use unfair, deceptive, high pressure sales tactics to scare up sales from gullible, confused users. And some companies have even been prosecuted by the government on those grounds. For more on these "rogue" anti-spyware products, click here...