Far be it from me to defend Microsoft, but I'm pretty sure their testing is complex, widespread and comprehensive. I've heard that it's tough to write more than a few lines a code a day because the infrastructure, bureaucracy, administration, testing, etc., etc., ad infinitum is just ridiculous.
Problem is, to say Windows is an astronomically HUGE program would be an understatement. It's literally millions of lines of code. Not to mention, the design is such that it's largely a house of cards. The slightest quaver and the whole bloody thing implodes and crashes into stasis. There's just no way to test it as thoroughly as it would need to be tested before shipping, unless you never want to ship it. Plus, the traditional IBM architecture it's designed to run on is an open spec, which means there are 3,419,442 ways to build a machine, 5 times that number of peripherals for it, and 10 times as many programs to run on it all written by people who don't talk to each other.
That it works at all is major miracle. It's been in beta for 30+ years now, and it's still not right. Windows 7 is probably the most stable thing they've ever shipped, and it's not without sin.