Spybot - Search and Destroy is a free malware and spyware detection and removal tool that utilizes open source malware definitions to protect your computer against attack. What makes the tool so useful is that it is very lightweight -- running in the background without interrupting your activities -- but it can actively identify threats when they appear on your computer.
Free and powerful: Unlike many spyware and malware detectors, Spybot is free for personal use, being supported largely by commercial licenses and donations. Combined with the frequent updates, speedy scans, and a number of tools like System and File Scan, Immunization, and a Quarantine-level update, this is a very powerful tool. While hidden at first, there are also advanced tools like Report Creator, Startup Tools, and Rootkit Scan.
Long pedigree: Spybot has been a free tool since the 1990's, and the developers continuously update and add new features. After installation, you'll be prompted to back up your Registry, and at any time you can switch from manual to automatic protection mode. For most users, then, Spybot is easy to use, running automatically in the background at set times, but it also offers more advanced tools for users with the experience to use them.
Some hidden tools: While all basic tools are free to use, and you'll never pay for a definitions update, some tools remain locked pending a paid upgrade. These include Phone Scan, Boot CD Creator, Secure Shredder, and System Repair. These are not vital or core parts of the software, but it's not immediately clear how to unlock and use them.
Some tools removed: Longtime favorite tools like TeaTimer have been removed, while new tools are available, albeit with some of them costing money. Spybot was never a full replacement for an Antivirus suite, but some longtime users may be frustrated with the changes.
If you have spyware or malware on your computer, your first step should be to download Spybot. This free detection tool is among the most trusted, and it's the ideal first step when you suspect a problem or want to ensure future problems don't create headaches.