Well, it depends on which version of Windows you are talking about. For the sake of keeping it short (and because I was very young at the time), I will leave Windows 1.0 and 2.0 out of it. Windows 3.1 was junk, it crashed all the time. Windows 95 was more stable but it still liked to BSOD when USB devices were connected. Windows 98FE was better and did not crash as much but it could be hard to deal with at times. Windows 98SE fixed some USB issues and it became more stable, personally I never had a problem with 98SE. Windows ME, probably the worst operating system ever invented, it crashed every five minutes without touching the keyboard. Windows 2000, now there was a robust operating system but then it was originally designed to run servers so it had to be reliable. Windows XP was built on Windows 2000 code so it is also very reliable. Again, personally, a Windows XP system has never gone south on me but take it for what you will, I'm a network security IT major so I kind of know what I'm doing.
If you had said has any operating system crashed as much as Windows ME, I would have said no. But in the case of Windows XP, in the four (five?) years I have been using Windows XP, not once has a system running it crashed on me. It also depends on the quality of the hardware in the system as bad hardware can BSOD a system faster than anything.
Also, leaving a computer on can fill up the kernel memory (memory leaks, applications that continue to run parts of themselves even though they are shut down) can cause problems which is why you should at the very least restart (a home PC) every 48 hours if you leave it on. I don't leave them on so it isn't a problem.