Antivirus or security software reports Steam is malicious

http://support.steampowered.com/cgi-bin/steampowered.cfg/php/enduser/std_adp.php?p_faqid=164

The Steam application has been mistakenly identified as a virus or "trojan" by certain software as a result of the development of potentially-malicious programs which use the same filenames as valid Steam files (this is a very common practice amongst malicious software writers - many viruses already masquerade as valid operating system files).

We are working with the antivirus and firewall software vendors to ensure fewer false positives involving Steam appear in the future.

In the meantime, please review the symptoms below to determine whether or not this issue is impacting your system:

Norton Firewall

When using the "Friends" feature, some Norton Firewall users have reported that their firewall software reports that Steam may be malicious software attempting to access their system. This is a false positive.

Occasionally the Friends server sends your computer a message asking if you're still online - if you closed Steam (especially if you killed it from the task manager), this message can be intercepted by the Norton firewall.

Since Friends sometimes uses the same port number as a known trojan horse virus, the firewall incorrectly identifies the message as being malicious - despite the fact that the traffic does not mimic a trojan in any other respect.