False Positive from Avast Labeled Steam as a Virus
"There was a very brief false positive issue which was corrected in 1.5 hours," a spokesperson for Avast told The Register.
False positives are an annoying, albeit largely necessary side-effect of keeping malware at bay. Typically, however, virus scanners are pretty good about not identifying safe and popular programs like Steam as potential viruses, but every once in awhile it happens, particularly if your security settings err on the side of caution.
"I was about to zero my HDD and start over! Updated Avast, backed up game caches, re-installed Steam. Everything works again," a gamer posted on Steam's forums.
To anyone who was/is affected by this, you should update your Avast definitions. You might also need to reinstall Steam if the accompanying executable was quarantined or deleted.