Spybot: Search and Destroy
Select the "Update" button to get the most up-to-date version of Spybot.
Search & Destroy
This button reveals the "Check for Problems" button, which scans the computer for various adware and spyware.
This automatically tweaks some internal settings to block the installation of known spyware (and similar threats) installers. This very useful feature will prevent many malicious programs from ever loading.