Just when it looked as if there might be some hope of cleaning up the nation's financial markets, sullied by conflicts of interest and fraudulent investment advice at major banks and brokerage houses, along comes evidence that another segment of the securities business has been rigged in favor of big investors at the expense of the little guy. This time the villains are mutual funds, usually deemed a safe haven for unsophisticated investors. In his latest investigation of tawdry practices in the financial markets, Eliot Spitzer, the New York State attorney general, revealed this week that some mutual funds had been making secret, illegal deals that benefited big in-and-out traders at the expense of legions of long-term investors. The practices, he said, are almost certainly widespread.