i would say if it's a clean reformat than it must be lurking in your steam or program files somewhere did ta save any of your steam files or down load any skins or werid shit recently or did ya go ta any werid links thou steam freinds ?? I would say completly wipe your steam install clean and start new (sure it's a pain but might solve the prob ) also the a slight chance your keyborad is fucked up try a different keyborad ( i have had keyborad and mouse probs errors before myself )

one other idea is that the prob is infeted in one of your emails or music or install files try checking all your install files

btw firefox and avg will warn ya in advance when ya going to a dangerous site or downloading a bad file