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:
Anti-Virus (AV) applications can utilize system resources that Steam games require to function properly. Since some AV software cannot release these resources, this may result in a system or game crash once your computer has run out of resources. It is recommended that you disable all AV programs before launching Steam to ensure all games have access to the system resources. Once finished playing these games, you can re-enable the software to ensure that the computer is protected.
Spyware, adware, viruses, 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 viruses 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.
Many commercial Anti-Virus software packages include much more than an Anti-Virus application. Packages such as Norton/Symantec and McAfee often include Internet Security components that have been known to interfere with Steam. Please disable your Anti-Virus software before launching Steam and re-enable it once you are done playing games.
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.