The first six months of 2010 have certainly been a wild ride for equities. After a nice start in January, stocks took a nosedive. By early February, investors were wondering if the bears would remain in control. Then the buying started again, and stocks surged from early February all the way through mid-April. However, since the April sell-off began, stocks have plunged precipitously. The S&P 500 Index (SPX) sank below its 50- and 200-day moving averages, and the broad measure of the market now trades at its lowest point since September 2009. Through the first six months of the year, the S&P 500 Index fell a frightening 7.57%.

Yet as bad as things have been for the broad market index over the past six months, things have been particularly gruesome for many of its high-profile companies. How bad have things been, and which companies qualify as the worst stocks through the first six months of the year? The following are the 10 worst-performing S&P 500 stocks so far in this very trying 2010.

Baxter International (BAX). The medical device maker’s shares have been rejected by investors over the past six months, with the stock selling off some 28%. The real catalyst for Baxter’s anemic performance was the company’s lowered 2010 sales and earnings guidance. The company cited slower growth in the plasma protein market as the reason for the reduced forecast. Then the company suffered from the announcement that it would take an additional charge of $400 million-$600 million following a recall of its Colleague pumps.

H&R Block (HRB). The tax-preparation firm’s shares took a 29% write-off in the first half of 2010. The lack of customers opting to do their taxes with the company—likely due to the high unemployment rate—caused a big decline in the companies fiscal Q3. In fact, the H&R Block said that the 2010 tax season witnessed the biggest drop in the number of filings with the IRS since 1971.

Boston Scientific (BSX). Another medical device maker that’s seen healthier days is Boston Scientific. The shares have fallen ill in 2010, plunging 35%. The company was forced to remove its implantable cardiac defibrillators (ICD) from the U.S. market after it failed to comply with FDA regulations. The ICD problem reportedly cost the company some $100 million, and could eventually force BSX to sell off its pain management division.

Diamond Offshore (DO). The oil and gas drilling firm’s shares tanked some 36% so far in 2010, thanks in large part to the disastrous BP plc (BP) oil spill. The spill prompted the Obama administration to impose a six-month moratorium on offshore drilling (a decision that’s since been overturned by a federal court), and one of the companies most affected by the ban was Diamond Offshore. Yet the legal fight over drilling in the Gulf of Mexico continues, with the Obama administration reportedly set to impose a revised ban on drilling in the Gulf in the coming days. So it should come as no surprise that investors are likely to continue being hesitant to embrace drilling stocks anytime soon.

King Pharmaceuticals (KG). The prescription drug maker’s shares took a depressing 37% downturn over the first half of the year. A disappointing Q1 earnings report that failed to meet Street expectations, along with a less-than-enthusiastic debut of its Embeda analgesic hurt the shares. Then there was the ruling against the use of Niacin in its Acurox pain relieving drug. If that weren’t enough to give King shareholders a major headache, the company also delayed resubmission of new dug application filing for its Remoxy pain-management therapy.

Monsanto (MON). The agricultural chemicals firm’s shares failed to bloom in 2010, with the stock down some 40%. Poor sales of the herbicide Roundup, and a continued legal battle with rival DuPont (DD) had investors fleeing the stock for more fertile farm land. Even a Supreme Court decision lifting a ban on planting genetically modified alfalfa seeds failed to really help the stock in the short term, although in the long run permission to plant the new alfalfa product could turn into a nice harvest for MON shareholders.

Anadarko Petroleum (APC). Another petroleum firm caught up in the BP imbroglio is Anardarko. The company’s shares sank 41% in the first six months of 2010, along with its 25% stake in the notorious Deepwater Horizon rig. The minority ownership in Deepwater means Anadarko will be subject to a hefty level of fiscal liability for the damages wreaked on the businesses and residents of the Gulf Coast region. Like Diamond Offshore, the company also faces an extremely uncertain future when it comes to continued operations in the Gulf.

AK Steel (AKS). The U.S. steelmaker’s shares forged a 43% decline in the first half of the year, as competition from Chinese steelmakers continues to put pressure on the entire industry. In addition to the increased competition is the rising price of iron ore, the critical raw material needed to produce steel. The combination of increased global supply and higher materials costs has really put pressure on AK Steel.

Nvidia (NVDA). The semiconductor chipmaker’s shares need a reboot after sliding 44% so far in 2010. Delays of its next-gen semiconductor chip helped slow down NVDA’s share price in recent months, and then the company lowered revenue guidance for the July quarter. That dour outlook really took its toll on the shares, and hence the horrible performance of the stock this year.

Dean Foods (DF). The worst performer of the first half of the year is dairy food producer Dean Foods. The stock fell 44% over the past six months, as a retail dairy product price war has really taken the cream off of the top of Dean’s margins. Then there was the sour Q1 profit shortfall of some 43%, with EPS that failed to come close to Wall Street expectations.

And one more for good measure…

BP plc (BP). Although not a member of the S&P 500, the now infamous oil company’s shares spilled nearly half of their value (48.6%) through the first six months of the year. While some might say it couldn’t have happened to a nicer outfit, it’s going to take more than a few relief wells to clean up investor confidence in this blown-up stock. And as we’ve already seen, BP is indirectly responsible for helping two S&P 500 companies make our worst performers list.

For the sake of these companies’ shareholders, let’s all hope the second half of 2010 is a lot different than the first half.