How do you define an OS as stable?
XP was stable in that you could do an honest day’s work without having to save it every couple of seconds in fear of a BSOD. Windows 2000 was stable: it gave the users what they expected, and once they got it working (driver issues mostly), it remained stable.
One can’t honestly call Vista stable in that way however. It’s largely a hit-or-miss process, and Vista either works or doesn’t. Once it works, it might just stop working, you never know. But Vista is stable – very stable.
In Windows XP (x86) if XP went up to over 26-27 thousand memory handles open at once, it would just fail. The entire operating system bogs down, and even after you get the handles down to a more manageable size, it remained slow and unresponsive unless a reboot was performed. Windows XP x64 used 64-bit technology to raise the bar to an amazing 35-38 thousand handles limit – from experience.