My main home domain is running from the original instance of the OS/domain done back in 2003, and has even survived a major machine upgrade along with a hard-drive failure in SYSVOL. My work machine is the same Win2000 Pro instance I originally installed in 2000.
Microsoft has done a remarkable job with their software robustness, as I have been 6 years since the point of no return - and have even survived major hardware upgrades.