「Just delete everything in the Steam folder except Steam.exe and the Steamapps folder.
Steam will recreate the files it needs. If that doesn't work, you may need to uninstall Steam and reinstall it.
Back up the Steamapps folder though or everything in it will be deleted.」
ってのがあるけどもう試した？
ようはスチームフォルダのSteam.exeとSteamappsフォルダ以外を全部消してSteam.exeを叩いてみればいいのかな？
http://forums.steampowered.com/forums/showthread.php?t=1436062
ウィルス関係の可能性もあるけどその辺はきちんとしてる？
見当違いの助言だったらごめんね
