Internet search leader Google is trying to convince federal and state authorities that Microsoft's Vista operating system is stifling competition as the high-tech heavyweights wrestle for the allegiance of personal computer users.  In a 49-page document filed April 18 with the U.S. Justice Department and state attorneys general, Google alleged that the latest version of Microsoft's Windows operating system impairs the performance of 'desktop search' programs that find data stored on a computer's hard drive.  Besides bogging down competing programs, Google alleged Microsoft had made it too complicated to turn off the desktop search feature built into Vista.