Mrs. Johnson and thousands of other computer programmers who want to work for themselves instead of being employees have run afoul of a 1986 law in which Congress decreed that most individual programmers cannot be entrepreneurs. The law generally excludes programmers from statutes giving employers some flexibility to use independent contractors. Critics say that the I.R.S. has recently stepped up its enforcement of the law in a way that effectively kills start-up programming businesses if their only employee is the founder.