Summary
Popup blockers are a good first step toward protecting against spyware/adware programs finding their way onto your computer.
Antispyware/antiadware programs offer protection against most spyware and adware threats. Some programs provide passive scanning (detection after infection), whereas others provide both passive and active scanning (detection before infection).
Much like antivirus technology, antispyware/antiadware programs rely on regular updates of signatures to be effective.