That's unavoidable. The last, best, most stable version of windows was Win95C. Since then, every new version of Windows is good, as far as they can test it on multiple platforms and multiple different setups. But multiple is limited. Release the OS and you go from multiple to exponential different pc's, with all sorts of different platforms apps, aged drivers, random bs games, specialized software etc. With the sheer amount on internet downloadable junk, there's absolutely no way MS can accommodate every single instance of code access, scripting, or forsee exactly how that'll affect the base OS code.
So they get the OS as good as they think it can be, then ship it worldwide and wait for the bug reports, user complaints etc to roll on, so they can address the issues.
Older versions of Windows only had to deal with AOL, Netscape, Yahoo and a handful of BBS. Now it's got to deal with the Pandora's Box that's the Internet of today.