The S&P 500 (SNPINDEX:^GSPC) pressed higher last week, closing at yet another all-time record high and more than making up for its losing performance the previous week. Even though concerns about Iraq and ongoing anxiety about the high valuations of the market still weighed on sentiment, economic strength continues to push stocks higher. In particular, company-specific issues helped make Covidien (NYSE:COV.DL), Williams Companies (NYSE:WMB), and CarMax (NYSE:KMX) the leading stocks in the S&P 500 for the week.

Covidien soared 25% as the Irish medical-device maker received a buyout offer from industry peer Medtronic, which involved a combination of cash and stock that represented an almost 30% premium to its price at the end of the previous week. Most analysts focused on the potential tax advantages for Medtronic in allowing the U.S.-based company to take advantage of more favorable tax laws abroad. But Covidien also offers a different yet complementary line of products that the combined entity could use to bolster its joint growth, and a larger company will be able to compete more effectively against the remaining players in the medical-device market. With hospitals and other health-care providers facing budgetary pressures and the challenges of health-care reform, Covidien was in the right place at the right time to receive this lucrative bid.

Source: Access Midstream Partners.

Williams Companies jumped 22% after the midstream company announced that it will acquire complete general-partnership control and a 50% limited-partnership interest in natural-gas gathering and processing specialist Access Midstream. The $6 billion cash deal will give Williams a commanding presence in most of the major shale-production regions of the country, including the Eagle Ford, Marcellus, Utica, and Niobrara plays. In addition, the larger scale of the combined Williams-Access entity should give it more leverage to make future acquisitions and make deals with exploration and production companies to bolster its growth.

CarMax climbed 19% in the wake of the used-car giant's latest earnings report. Solid sales and earnings made investors more confident than ever in the company's future, but even more importantly, CarMax demonstrated substantial growth in its online business. Given that a large part of CarMax's appeal is in offering hassle-free car purchasing, the e-commerce venue is particularly well-suited to its customer base. If CarMax can demonstrate that it can make online buying even more painless than its dealerships, then it could end up pioneering a new way for consumers to buy vehicles.

Despite trading at record highs, the S&P 500 has many investors looking forward to the possibility of breaking the 2,000 milestone in the near future. Watch closely to see if these three stocks can keep leading the index higher in the weeks to come.