All the time. I have had Win XP machines running for more than a year. Of course it poor thing has to be behind a good firewall, such such uptimes does mean it isn't allowed to automatically retrive updates and then reboot.

In the end, almost all instabilities are caused by bad drivers, or by a cheap motherboard.

A big problem with Windows is that the drivers model is so complex - it is way harder to write a good driver for Windows than for Linux. And most hw companies has all their skills in the hw department. As soon as the hw works, they want to ship the hardware and start designing the next hardware. The end result is often strange behaviours or BSOD.