Martin Fowler writes about Very Low Defect Projects:

I was particularly happy to run into the second case I saw - since it involved several of my old friends from C3. They are now building some portal software at Chrysler. Although they started off at the shocking defect rate of one a month, during 2002 they recorded exactly one bug against their system. During this time they were releasing new version of their software every week or two.

He's careful to point out that adopting XP won't automatically lower your bug count rate to barely detectable levels, but maybe it helps.