Windows XP. Hasn't crashed in two years. Kept lean and clean with CCleaner, AVG, and WinPatrol. No updates to any programs without permission and necessity. Nor any need to upgrade for a while yet.
Four HDs in all, 2.8 Tb, main one is kept never more than half full, programs removed if not used in six months.
All working docs, files, pictures, etc stored on separate internal HD. Weekly backup of full system to external HD using Acronis True Image. Much easier to use than Norton Ghost, or whatever the current version is called. Plus also docs and pictures copied weekly to another HD. Then 6 monthly copies of docs and photos onto DVDs. Sounds cumbersome but a routine that takes little time with practice - apart from burning DVDs!
(Typing this while John posted his chaser.)