I was running XP SP3 (and the old system still exists but is now for either DVD authoring or Dolby Digital encoding since the DDE will install on Windows 7 but is not recognized in Nuendo any longer) and stability was never an issue for me at all since the days of version 2.
N6 will definitely not install on XP (although I suspect this is a conscious choice) so now I am running both 5.5 & 6 on Windows 7 with no issues at all as far as stability goes.
It has been my experience that 99% of stability issues are caused by incorrect usage, bad setup, mismatched components, bad drivers or 3rd party plugins…