Spybot Search & Destroy was once considered best of breed, and seems to have rested on its laurels ever since. Having neither real-time protection, automatic updating, nor scheduled scanning, I cannot imagine it being used for anything other than a second opinion tool for advanced users. It is not open source but is available at no cost for private use.
INSTALLATION AND SETUP
Download the latest stable version of Spybot. Once downloaded, right-click on the installer and select “Run as administrator”, which is the proper way to install any security software. Make sure you remain connected to the Internet, as the installer will require it. Unless the end-user is unusually sophisticated, disable the Tea Timer option during installation (enabled by default).
Once installed, launch Spybot and follow the first time wizard. Then set the end-user language if required using the “Language” entry on the menu.
Setting up automatic updates or scheduling periodic automatic scans is a clumsy and unreliable process described in this forum discussion.
Run Spybot from the start menu. The Spybot website has an excellent tutorial. Real time protection is not available.
On the basis of this and many similar negative reviews, I no longer recommend Spybot to my clients.
Is Spybot not what you’re looking for? See my list of other spyware scanners.