It really depends on your usage profile. If you are installing/uninstalling new software very often, the system will sooner or later became corrupt or bloated/unusable. Users with office usage profile won't see this problem. But I studied computer science + I used to be an avid gamer, which means that I always added some new software (games, programming utilities, stuff I wanted to check out etc.) to my computer. Windows XP would either become a memory hog or blatantly refused to boot approximately once in two month. Of course, it is probably possible to restore the installation without reinstall, but in fact, the reinstall is the smarter way to deal with it, as its often safer and quicker. Since Vista the MS OS is very stable though, not any worse than OS X.