Microsoft Corp. is about to complete the biggest change in five years in the way that it sells its software to businesses with a system of locked-in upgrades and fixed payments that promises steadier revenue but has also rankled some smaller customers. By Wednesday, the world's No. 1 software maker will have fully implemented the change in the way business customers pay for the right to use the latest versions of its software. Chief Executive Steve Ballmer admitted that the shift in Microsoft's complex volume licensing practices, which it introduced five years ago, had sown some confusion.

"The fact that our customers probably didn't understand our licensing as well they might have earlier makes the transition and the perceived pain higher than it actually is," he told analysts at the company's Redmond, Washington headquarters last week. "So we're smarter for the experience, that's for sure." Microsoft hopes to stabilize its income through multi-year contracts that promise to deliver regular updates through the new "Software Assurance" program.