A: From my experience using both Linux and Windows, Windows XP seems to be the better of the two so far. Claims? Definitely usability and stability. X Windows/KDE programs seems to crash a little more often (maybe that's because of the program and not the OS). I'm sure Linux fans would say otherwise.
A: Depends how much stuff and how well your system is running. I recommend installing Norton System Works so you don't need to re-install/re-format Windows.
A: There's a bunch of utilities that comes bundled with Norton System Works. The main program which provides a level of system stability is Norton Utilities/Norton SystemDoctor/WinDoctor, which basically checks your registry and other system settings and fix any problems that it finds.