* Windstream rallies; set to spin off assets into REIT
* Pfizer, Merck rise as results beat expectations
* Herbalife shares tumble after it cuts sales outlook
* Futures up: Dow 40 pts, S&P 3 pts, Nasdaq 7 pts (Updates prices, adds data)
By Rodrigo Campos
NEW YORK, July 29 (Reuters) - U.S. stocks were set to rise slightly at the open on Tuesday following better-than-expected results from companies such as Pfizer and Merck and ahead of data on consumer sentiment.
* Merck reported better-than-expected quarterly results as new drugs offset declining sales of ones facing generic competition. Pfizer's higher-than-expected second-quarter earnings were helped by growing sales of its cancer medicines. Merck was up 1 percent in premarket, while Pfizer gained 0.6 percent.
* Shares of UPS fell 3 percent premarket after the world's biggest courier company slashed its earnings forecast for the year as it spent to boost capacity.
* Single-family home prices fell 0.3 percent in May on a seasonally adjusted basis, falling short of expectations calling for a 0.2 percent gain. The Conference Board's consumer sentiment for July is due at 10:00 a.m. EDT (1400 GMT).
* Later on Tuesday, the Federal Reserve begins a two-day meeting on monetary policy. The Fed could make subtle yet telling changes to its policy statement due Wednesday, as it plans how and when to eventually raise interest rates.
* S&P 500 e-mini futures were up 3 points and fair value - a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract - indicated a slightly higher open. Dow Jones industrial average e-mini futures rose 40 points and Nasdaq 100 e-mini futures added 7 points.
* Shares of Windstream Holdings jumped 31 percent in premarket trading after the company filed to spin off assets into a publicly-traded real estate investment trust.
* Shares of health insurer Aetna fell 2.6 percent before the opening bell, even as it reported a higher second-quarter profit, helped by last year's acquisition of Medicare and Medicaid provider Coventry Health Care.
* Spirit Airlines rose 2.6 percent in light premarket trading after the low-cost U.S. carrier reported a 54 percent rise in second-quarter profit.
* Costco shares rose 2.2 percent premarket after Goldman Sachs added the stock to its conviction buy list.
* Herbalife cut its sales outlook for the year after raising it three months earlier, following billionaire investor Bill Ackman's latest round of accusations against the company last week. Shares of the weight-loss and nutrition products company slid 12 percent in premarket trading. (Editing by Bernadette Baum)