Spyware, malware, or adware is interfering with Steam's functionality.
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. If you need help creating an exception, please contact the security software's support team for specific instructions.
When searching for or choosing a malware removal tool, make sure to 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.