Ad-aware

From Lavasoft:

With its ability to scan your RAM, registry, hard drives, and external storage devices for known data-mining, advertising, and tracking components, Ad-aware can easily clean out your system, allowing you to maintain a higher degree of privacy while you surf the Web.

The redesigned user interface improvements include more flexibility and control. This includes user-defined file sizes, which speeds up the scanning process by excluding files it recognizes as clean. Other advanced features include automatic quarantine and Recycle Bin storage of removed components, an all-new viewer for backup archives that allows you to quickly and easily review components that have been removed and stored for future restoration, and an integrated reference file updater that can automatically search for new reference files when Ad-aware is started. The latest version includes improved Browser Hijacker detection, hosts file scanning, and a Smart Scan mode that improves speed and has been optimized to detect content at the root. Be aware that some ad-supported programs, such as GoZilla Free, will no longer function if you remove their ad modules.