The antitrust people would start off by looking at the hypothetical merged companies percentage of the relevant market. If the the relevant market is defined as "search," MSFTHOO would create an entity with only a 30% of the market compared to GOOG's 65% (based on numbers from here: http://www.hitwise.com/press-center/hitwiseHS2004/searchengi...). This is not to say the FTC or DOJ wouldn't sandbag such a deal, but how do you gain monopolisitic market power or anti-competitive effects from a 30% market share? I guess you could keep blowing up the definition of the relevant market until you captured MSFT's entire business but that seems dubious.