Spyware, malware, or adware is interfering with Steam's functionality. How do I...
If your computer has become infected with malware/adware that targets Steam, you may see one or more of the following issues:
Spyware, adware, virus, trojans and worms may directly impact your ability to access Steam - if you have exhausted other troubleshooting steps, or have noticed unexpected behavior and decreased performance on your machine, please download and install a spyware and adware scanner and a virus scanning utility.
The most common virus are transmitted through e-mail attachments, downloading files from resources that are not entirely trusted, or peer-to-peer (p2p) file trading applications such as BitTorrent. With any Anti-Virus program, it is important to keep Anti-Virus definitions updated and run a system scan regularly.
A common source of Spyware/Adware is installing unknown software from untrusted websites. These programs can cause system slowdown and/or connectivity problems.
Some malware removal tools may interfere with Steam - make sure that you set up the application appropriately and create exceptions for Steam and its games.
When searching for or choosing a malware removal tool, ensure that you research the tool before making a decision. Download the tool only through official, trusted websites. Many self-described malware removal tools are actually malware themselves.