Our always-connected computers are, by the same token, always vulnerable. So it would be foolish not to take the negligible performance hit and run decent antivirus software. Our favourite freebie, ignoring its frequent pleas to upgrade to the paid version, is Avira.
It uses a cloud-based network to ensure all of its users are protected against the latest nasties as soon as a new one crops up in the wild, it can discover trojans and other malware hidden in legitimate apps, and has a straightforward, easy-to-use interface. And, most importantly, it hit a 100% detection rate on AV-Test's latest round of antivirus punishment, so you can rest relatively easy.