adaware free antivirus is a popular malware hunter.
Install the program and it goes to work immediately, monitoring running processes and the files you're accessing to block threats before they can do any harm.
You can also run various types of scans on demand, either from the program or by right-clicking a file in Explorer and selecting "scan with adware..."
The free build of the program has some major limitations. In particular, there's no online protection apart from the scanning of downloads, and no scanning of emails.
Upgrading to a commercial edition will fix this and add other features.
Adaware Pro extends the free build with web, online banking, email and network protection, and a firewall.
Adaware Total also gives you encryption, secure file deletion and parental controls.
Adaware is easy to use, but doesn't have the best of detection records.