Spring has sprung and so has the market. Like the first crocuses of the season popping their pretty little heads out of the dirt, Dow 8000 is a beautiful sight for winter-weary investors.

But why the optimism now? It's been a bleak two months since the Dow Jones Industrial Average last saw this level and -- even worse -- that was on the way down. This may be just another bear-market head-fake, but there's no denying that the Dow and the broader S&P 500 are up more than 20% from their March 9 closing lows. And, hey, that's certainly better than the alternative of further declines.

Though there are plenty of challenges ahead -- including what promises to be a dismal unemployment figure later this week -- but at least this rally isn t built entirely on thin air. For the first time in a long time, economic and corporate news is starting to come in better than expected. Financial institutions caught a break in how they account for troubled assets. And, most important, the new administration is -- for the time being -- re-instilling confidence on Wall Street and Main Street.

Here, then, is a look at the five pillars of a most welcome rally.

Meetings rarely fix problems but they re often freighted with big hopes, and the market reaction to this week s gathering of the G-20 heads of state in London is no exception. The most important leaders in the world have put their heads together, and if they haven t come up with definitive answers to the world s economic problems, at least they re giving the appearance of a coordinated response. That goes a long way toward bolstering investor confidence through the world.

The group responsible for U.S. accounting standards gave the OK to a long-sought change in accounting rules that will let banks use looser criteria in determining the worth of their hardest-to-value assets -- sending shares of financial-services companies up sharply. The debate over mark-to-market accounting has raged since well before the worst of the financial crisis, but now the Financial Accounting Standards Board has relaxed its terms. The bottom line: Banks' liabilities from so-called toxic assets (which have hobbled many financial firms and exacerbated the credit crisis) will now weigh less heavily on their balance sheets.

There's no doubt that the Obama administration stumbled a bit after the inauguration as it failed to fully explain the details of its plans to get the economy back on track. But in recent weeks the president, Treasury Secretary Tim Geithner and the rest of the team have hit their stride. Whether it s TARP, TALF or any other plan with an acronym, investors now have a clearer sense of how these things will work. The market has "cheered the recent actions taken by the Federal Reserve and U.S. Treasury, says Bill Stone, chief market strategist at PNC. That was certainly the case with the Public-Private Investment Program, the $1 trillion plan that helped stoke this rally two weeks ago with a 497-point pop.

There was a blue-light special on blue-chip stocks, with prices far too low for investors to ignore. When the market started to rise, investors and money managers feared being left behind on giant moneymakers like DuPont (DD), up about 11% so far this week; Disney (DIS), up 9%; and Microsoft (MSFT), up 7%. At the beginning of March, all three traded at bargain-basement valuations of less than 10 times expected earnings.

Take roughly $14 trillion in cash sitting on the sidelines. Add in the reality that the market trades on perception as much as reality. And then throw in a few critical historical market facts: Within six months of a new bull market, more than a quarter of the gains have already been booked, according to research from Fidelity Investments, while more than 40% of the gains come within the first year. Meanwhile, Standard & Poor's has found that investors on average recoup 80% of their bear-market losses within the first year of the next bull. Maybe this isn't the beginning of the next bull market, but given those stats, investors can't afford to miss out on the stampede.