the reason is that Operation System is a very difficult program, it involves concurrency. it is very diffcult or even impossible to make sure a concurrent program to be completely error-free due to the parallel execution.
Windows are getting more and more complex with its functionalities and appearance as new version releases. With higher complexity of the program, it becomes even more difficult to program error-free concurrency code. therefore, u see xp having a lot more bugs than previous windows (xp is like a revolution to previous windows, way more complex) and vista even more bugs (vista is another revolution, way way more complex).
Moreover, when microsoft creates new version of windows, most likely they won't create them from scratch, they will use existing code from previous windows, and when u think about it, windows are created back in the 80s, and back then, computer system is much more simple, they don't have to worry much about the diffculty in executing programming code in a multi-cpu parallel environment when they program the code. thus, the existing code that they re-use in newer version will not be perfectly compatible with the current parallel systems.
you will probably be wondering about linux for being much better even though it's also an OS which suppose to be a diffcult program, but u should note that linux is actually a very simple OS, it executes programs in console manually. bugs will be everywhere if it gets as complex as vista

So, concurrent programming is the reason behind the bugs
Note that concurrency is still a very immature concept, programming language is still evolving to comprehend that, so we will yet to see an actual good OS for modern computers to be produced