Recently I installed Kaspersky Internet Security 2010 onto my Windows 7 x64, and it replaced the built-in Windows firewall (there was an option to do so while installing, and I deliberately chose it). Everything was fine except the next day I wasn’t able to launch Steam, which complained with an error “Steam is temporarily unavailable” while trying to update.
None of the solutions suggested by the Google search worked for me (including repairing and reinstalling Steam, just deleting some .blob files, etc), so having considered the installation of Kaspersky as the only possible influence factor, I checked Windows firewall configuration in the Control Panel, deleted Steam outta there and re-added it as a safe application.
This solved the problem. (It wasn’t too obvious: all other programs were working fine, Windows did not complain and Steam was a “trusted” program in built-in firewall settings).