if you run antivirus software, how do you know you're not infected? many malwares now will partially paralyze your antivirus so that it seems all is well, but your computer is operated by someone else. TDSS would do this as long ago as 2008...

You can use services like VirusTotal to do a quick file check if you suspect anything. You can also check what services/program starts on boot (msconfig and services.msc). With that you can already have a good confidence level of not being infected without have an AV draining your computer performance.