Neither XP, Vista nor W7 (and probably W10 - not much data so far) will crash itself on good hardware (including good drivers). My vista64-at-work did run two years even without logging off 🙂 I have seen crashes on my and friends PCs due to the following reasons:

bad/unstable/cheap/'tired' PSU (tired means that capacitories were dead, multiple cases) tired motherboard (capacitories, multiple cases - first indication is cold-start problems) bad or overclocked or cheap memory (test failed even at factory settings) bad or nonexistent cooling (cpu/chipset overheating problems) bad video chip (some batches of NVidia mobile GPU, problems arised after year or two) bad video drivers (mostly on older systems, could be coupled with PSU problems, many problems with old NVidia cards) bad video recording software/drivers (on my old ATI-AIW, only system restore helped) bad network card drivers (on older systems, some realtek 8029? drivers were buggy) bad system-level software drivers (antivirus, game protection, game protection hacks) corrupted hard disk or file system

What about crash on opera closing, then this may indicate memory problems (on closing a lot of memory gets reallocated/freed), disk problems (cache/temp files deallocating and settings saving - is your hdd light on while crashing?), video driver problems (opera uses hardware graphic acceleration when possible - does your screen go blank or lock up before restarting?), network problems (connection closing) - not easy to find out actually.

Have you noticed any other problems or unexpected behavior on your PC?