The Microsoft Windows Malicious Software Removal Tool is a quick and easy tool to uncover and eliminate malware from your system. Just choose the type of scan you need - quick (takes a few minutes), full (takes ages, usually) or custom (scan only the areas you specify) - then leave the program to check your hard drive for threats, and remove any that it finds.
While useful, this is no substitute for an antivirus package. The Malicious Software Removal Tool has no real-time detection features, for instance, so it can't stop you getting infected in the first place. And it only checks for some of the most common malware (see the full list here), currently around 150 specimens in total) - a good antivirus tool will detect hundreds of thousands of threats.
Still, if you think your PC might be infected by something, but your current antivirus suite hasn't raised the alarm, then you can always try the Malicious Software Removal Tool for a quick second opinion (it won't clash with any existing security tools).
And because the program doesn't require installation, it's ideal for including on a USB flash drive as a part of your portable troubleshooting toolkit. Then, if a friend's PC seems to be having virus troubles, you can pull out the Malicious Software Removal Tool and give their system a speedy health check.