Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

The S&P 500 Index (INDEX: ^GSPC) approached record highs on Monday on the heels of Larry Summers' decision to withdraw his name from consideration as Federal Reserve chairman. The new front-runner, Janet Yellen, is seen as a more accommodative central bank leader, more inclined to continue monthly stimulus injections to keep the economy going. The S&P added 9 points, or 0.6%, ending at 1,697. Despite the uptrend, these three stocks stubbornly refused to rise, dragging on the benchmark index. 

Not only is Apple (NASDAQ: AAPL) the single largest company in the S&P 500, it was today's single biggest decliner as well. The 3.2% slump today follows up last week's meltdown, when investors heavily sold shares after the company revealed its new iPhone 5s and 5c models and its decision today to not announce preorder figures, along with reports that China Telecom won't offer the subsidies shareholders had been expecting.

Apple's headline-making pullback has taken the entire tech sector down with it, and Hewlett-Packard (NYSE: HPQ) felt the hit, falling 1.5%. HP is being dropped from the Dow 30 at the end of the week, a decision made in part because its stock price has fallen to the point where it's one of the three weakest-weighted components in the index. With consumers shifting more and more to tablets and competitor Dell going private, HP faces some significant headwinds moving forward.

Lastly, Valero Energy (NYSE: VLO) fell 1.4%. Shares in the oil refiner have been pummeled in the past week as refiners underperformed in the face of rising oil prices and pressured margins. Valero in particular could also be hit by the increasing popularity of Renewable Identification Numbers, or RINs, as investment vehicles for banks, a trend that the company says could drive up costs by $800 million. 

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Fool contributor John Divine owns shares of Apple. You can follow him on Twitter @divinebizkid and on Motley Fool CAPS @TMFDivine.

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