Microsoft has launched a new (free) virus/malware scanner called Microsoft Security Scanner (MSS). It's designed to be used in emergencies, if you think your computer might be infected by a virus that your regular security software didn't detect.
To use MSS, you download the file here and then run it on your PC. The app is portable, so it doesn't need an installer. When you run it, it will analyze your computer and detect (and remove) many of the most prevalent hacker surprises.
Please note, however, that this MSS download automatically expires after 10 days. If you need to use it again after that, you'll need to keep downloading the latest version when the need arises.
OK, you say, that doesn't seem so bad. But MSS weighs in at 67 MB--and you're expected to download it each time you need it!
You could choose to rely on MSS anyway-- or you can turn to McAfee, which offers a similar product (also free) called Stinger. Stinger doesn't expire after 10 days-- and it weighs in at just 7.7 MB. As they say, the choice is yours-- but I know which product I'll be choosing...