The Free edition of a-squared is one of the best malware scanners, which you don't have to pay for.
a-squared Free 3.0 (picture: emsisoft.com)
The scanner can't find viruses though, but all the other artificial vermin, like Trojans or spyware. The gratis software a-squared sticks to the basics: Background scanning and automatic update function aren't included and you have to start the scanner manually.
You can use a-squared with Windows Vista, too. In June the new version 3.5 was released, but nearly every day there are additional updates - see the changelog for detailed information. You can download the tool at Emsisoft.