US STOCKS-Dow set for 2011's best month but Microsoft drags

* Caterpillar leads industrials higher on results, outlook * Microsoft falls on results, RIM tumbles after outlook * Merck edges higher after results * Dow up 0.4 pct, S&P up 0.2 pct, Nasdaq up 0.01 pct * For up-to-the-minute market news see STXNEWS/US (Updates to late afternoon, changes byline)