"I'm proud of what we've achieved, but I'm not content," the president said. "I'm pleased with our progress, but I'm not satisfied."
He went on to detail the challenges facing the country, including lost jobs, a deficiency of credit, the struggles of the auto industry, long-term deficits, and "threats ranging from terrorism to nuclear proliferation, as well as pandemic flu."
"All this means you can expect an unrelenting, unyielding effort from this administration to strengthen our prosperity and our security in the second hundred days, in the third hundred days, and all of the days after that," he said.
The president warned Americans not to expect to return to the practices and lifestyles that preceded the economic downturn.
"Even as we clear away the wreckage of this recession, I've also said that we can't go back to an economy that's built on a pile of sand, on inflated home prices and maxed-out credit cards, on overleveraged banks and outdated regulations that allow recklessness of a few to threaten the prosperity of all," he said.
The president opened the news conference by commenting on the swine flu situation - or, as the White House now calls the disease, the H1N1 flu virus.
"Every American should know that their entire government is taking the utmost precautions and preparations," he said.
"I've asked every American to take the same steps you would take to prevent any other flu: keep your hands washed; cover your mouth when you cough; stay home from work if you're sick; and keep your children home from school if they're sick," he added.
The first question posed to Mr. Obama concerned "under what conditions you might consider quarantining." The president responded that "this is a cause for deep concern, but not panic" and said he is following the advice of health officials who are not recommending closing the U.S.-Mexico border.
"From their perspective, it would be akin to closing the barn door after the horses are out, because we already have cases here in the United States," he said. (Read more about the president's comments on the flu outbreak.)
Asked about immigration, the president said the country "can't continue with a broken" system. He said he believes his former rival for the presidency, Sen. John McCain, has the "right position on immigration reform" and said he " would love to partner with him and others on what is going to be a critical issue."
The president also addressed the plight of the struggling U.S. auto industry, offering words of encouragement for Chrysler and General Motors.
"I am actually very hopeful, more hopeful than I was 30 days ago, that we can see a resolution that maintains a viable Chrysler auto company out there," he said. On GM, the president said he believes "they can emerge a strong, competitive, viable company."
Watch President Obama's press conference, in full:
"I would love to get the U.S. government out of the auto business as quickly as possible," he said. "And it was my obligation and continues to be my obligation to make sure that any taxpayer dollars that are in place to support the auto industry are aimed not at short-term fixes that continue these companies as wards of the state, but rather institutes the kind of restructuring that allows them to be strongly competitive in the future." (Read more about the president's comments on the auto industry.)
Asked whether "the previous administration sanctioned torture," the president said, "I do believe that [waterboarding] is torture." He said he is "absolutely convinced" that ending such practices was the right thing to do because it "corrodes the character of a country."
The president also suggested that his administration might back away from its controversial decision to adopt a Republican legal strategy aimed at derailing lawsuits alleging government wrongdoing. (Read more about the president's comments on state secrets.)
And he said he is "confident that we can make sure that Pakistan's nuclear arsenal is secure."
"We want to respect their sovereignty, but we also recognize that we have huge strategic interests, huge national security interests in making sure that Pakistan is stable and that you don't end up having a nuclear-armed militant state," said Mr. Obama.
Not every moment in the press conference was quite so serious; asked what most surprised, enchanted, humbled, and troubled him in office, the president quipped that he needed to write down all the categories. When he got to "enchanted," he smiled and said, "enchanted. Nice." (Read the entire exchange.)
Though he remains popular (Mr. Obama's approval rating in the most recent CBS News/New York Times poll) and can claim significant victories in his first 100 days, the president is pushing an ambitious agenda that includes rejuvenating the economy, overhauling the health care system, and dealing with two wars and other foreign policy challenges.
He got an unexpected boost in getting that agenda through Congress from Pennsylvania Sen. Arlen Specter, who announced Tuesday that he had decided to become a Democrat. The decision creates the prospect of a filibuster-proof majority in the Senate for the Democrats if and when Minnesota Democrat Al Franken is seated.
The president welcomed Specter to his party Wednesday morning, saying he was "thrilled" about the decision and calling the longtime Republican "one tough hombre."
CBS News Chief White House Correspondent Chip Reid asked the president what the move says about the state of the Republican Party and the notion of "one party rule."
"I am under no illusions that suddenly I'm going to have a rubber-stamp Senate. I've got Democrats who don't agree with me on everything, and that's how it should be," he said.
The president was also asked about the government's role as a shareholder in auto companies and other businesses that have taken federal money.
"You know, I don't want to run auto companies," he said. "I don't want to run banks. I've got two wars I've got to run already. I've got more than enough to do. So the sooner we can get out of that business, the better off we're going to be."
"I'm not an auto engineer," he added. "I don't know how to create an affordable, well-designed plug-in hybrid. But I know that, if the Japanese can design an affordable, well-designed hybrid, then, doggone it, the American people should be able to do the same."
"This was really President Obama at his best tonight," said CBS News Chief Washington Correspondent Bob Schieffer following the news conference. "He was to the point, he said what he wanted to say…and in the end I think he probably didn't tell us a single thing he didn't want us to know."
In his opening statement, the president once again lauded his $787 billion stimulus bill, which he said "has already saved or created over 150,000 jobs and provided a tax cut to 95 percent of all working families."
He mentioned other accomplishments as well, including the passage of legislation providing health insurance for "eleven million American children whose parents work full time" and "a housing plan that has already contributed to a spike in the number of homeowners who are refinancing their mortgages, which is the equivalent of another tax cut."
The budget "contains new investments in education that will equip our workers with the right skills and training, new investments in renewable energy that will create millions of jobs and new industries, new investments in health care that will cut costs for families and businesses, and new savings that will bring down our deficit," he said.
At a Missouri town hall meeting earlier Wednesday, the president struck a similar tone.
"We have a lot of work left to do," the president told reporters at the news conference. "It's work that will take time, and it will take effort. But the United States of America, I believe, will see a better day. We will rebuild a stronger nation, and we will endure as a beacon for all of those weary travelers beyond our shores who still dream that there's a place where all of this is possible."
The Obama administration initially dismissed the "100 Days" milestone as what senior presidential advisor David Axelrod called a "hallmark holiday." But the administration eventually embraced the benchmark, which grew out of Franklin D. Roosevelt's busy first three-and-a-half months in 1933.
Click here for complete 100 days coverage with more reaction and analysis.