Metascan Client is a portable tool which can scan running processes for malware.
The program is extremely simple. Just launch it, click Scan, and it'll calculate hash values (digital fingerprints) for all running processes and their modules. These are sent to the Metascan server, which uses multiple antivirus engines (a subset of ESET, AVG, Microsoft , Bitdefender, Symantec, F-Secure, GFI, Kaspersky, and McAfee) to look for threats. If the hash isn't recognised then the file is uploaded, and when the scan is complete you'll be told the results.
There's no online protection here, then. No full system scan. No scheduled scans, and not even the ability to remove anything it finds, so Metascan Client is no substitute at all for a real antivirus tool.
If you think you might be infected and your regular security package hasn't raised any alerts, though, the program is at least a quick and easy way to get a second opinion. And as it's also entirely free we'd recommend you download a copy, just for emergencies.